CF#252 (Div. 2) B.

alera and Fruits

time limit per test

1 second

memory limit per test

256 megabytes

Valera loves his garden, where n fruit trees grow.

This year he will enjoy a great harvest! On the i-th treeb_i fruit grow, they will ripen on a day numbera_i. Unfortunately, the fruit on the tree get withered, so they can only be collected on daya_i and daya_i + 1 (all fruits that are not collected in these two days, become unfit to eat).

Valera is not very fast, but there are some positive points. Valera is ready to work every day. In one day, Valera can collect no more thanv fruits. The fruits may be either from the same tree, or from different ones. What is the maximum amount of fruit Valera can collect for all time, if he operates optimally well?

Input

The first line contains two space-separated integers n andv(1 ≤ n, v ≤ 3000) — the number of fruit trees in the garden and the number of fruits that Valera can collect in a day.

Next n lines contain the description of trees in the garden. Thei-th line contains two space-separated integersa_i andb_i(1 ≤ a_i, b_i ≤ 3000) — the day the fruits ripen on thei-th tree and the number of fruits on thei-th tree.

Output

Print a single integer — the maximum number of fruit that Valera can collect.

Sample test(s)

Input

2 31 52 3

Output

8

Input

5 103 202 201 204 205 20

Output

60

Note

In the first sample, in order to obtain the optimal answer, you should act as follows.

• On the first day collect 3 fruits from the 1-st tree.
• On the second day collect 1 fruit from the 2-nd tree and 2 fruits from the 1-st tree.
• On the third day collect the remaining fruits from the 2-nd tree.

In the second sample, you can only collect 60 fruits, the remaining fruit will simply wither.

解题思路：

        题意是说，先输入n和m，代表n棵树和每天最多能收获m个水果，接下来n行，每行两个数a和b，代表每棵树的成熟日期

和水果数。规定每棵树采摘只能在成熟日 i 和 i + 1 两天进行，过期扔掉。最后求最大收获多少水果。

        用数组模拟，vis的下标代表天数，vis[ i ]代表第 i 天成熟的水果数量（可能有多棵树在同一天成熟），然后循环遍历，如果

vis[ i ] > m ，那么sum += m ， 然后vis[ i ] -= m，接下来进行第二次判断。如果第i + 1 天vis[ i ]仍旧大于m，那么把vis[ i + 1 ] += m;

否则vis[ i + 1 ] += vis[ i ]。举例来说，如果m = 10,vis[ i ] = 30 ，那么在第一次判断完成后，vis[ i ]变成20，还是大于10，那么第i + 1

天就可能没有树成熟，那我们就可以在第i + 1天继续收第 i 天的水果，能收的最大限度为m，即10。

      如果第一次判断就不满足条件的话，那直接sum加上vis[ i ]就好。

AC代码：

#include <functional>#include <algorithm>#include <iostream>#include <fstream>#include <sstream>#include <iomanip>#include <numeric>#include <cstring>#include <climits>#include <cassert>#include <complex>#include <cstdio>#include <string>#include <vector>#include <bitset>#include <queue>#include <stack>#include <cmath>#include <ctime>#include <list>#include <set>#include <map>//#include <tr1/unordered_set>//#include <tr1/unordered_map>//#include <array>using namespace std;#define REP(i, n) for (int i=0;i<n;++i)#define FOR(i, a, b) for (int i=a;i<b;++i)#define DWN(i, b, a) for (int i=b-1;i>=a;--i)#define REP_1(i, n) for (LL i=1;i<=n;++i)#define FOR_1(i, a, b) for (int i=a;i<=b;++i)#define DWN_1(i, b, a) for (LL i=b;i>=a;--i)#define REP_C(i, n) for (int n____=n,i=0;i<n____;++i)#define FOR_C(i, a, b) for (int b____=b,i=a;i<b____;++i)#define DWN_C(i, b, a) for (int a____=a,i=b-1;i>=a____;--i)#define REP_N(i, n) for (i=0;i<n;++i)#define FOR_N(i, a, b) for (i=a;i<b;++i)#define DWN_N(i, b, a) for (i=b-1;i>=a;--i)#define REP_1_C(i, n) for (int n____=n,i=1;i<=n____;++i)#define FOR_1_C(i, a, b) for (int b____=b,i=a;i<=b____;++i)#define DWN_1_C(i, b, a) for (int a____=a,i=b;i>=a____;--i)#define REP_1_N(i, n) for (i=1;i<=n;++i)#define FOR_1_N(i, a, b) for (i=a;i<=b;++i)#define DWN_1_N(i, b, a) for (i=b;i>=a;--i)#define REP_C_N(i, n) for (int n____=(i=0,n);i<n____;++i)#define FOR_C_N(i, a, b) for (int b____=(i=0,b);i<b____;++i)#define DWN_C_N(i, b, a) for (int a____=(i=b-1,a);i>=a____;--i)#define REP_1_C_N(i, n) for (int n____=(i=1,n);i<=n____;++i)#define FOR_1_C_N(i, a, b) for (int b____=(i=a,b);i<=b____;++i)#define DWN_1_C_N(i, b, a) for (int a____=(i=b,a);i>=a____;--i)#define ECH(it, A) for (__typeof(A.begin()) it=A.begin(); it != A.end(); ++it)#define REP_S(i, str) for (char*i=str;*i;++i)#define REP_L(i, hd, suc) for (int i=hd;i;i=suc[i])#define REP_G(i, u) REP_L(i,hd[u],suc)#define REP_SS(x, s) for (int x=s;x;x=(x-1)&s)#define DO(n) for ( int ____n = n; ____n-->0; )#define REP_2(i, j, n, m) REP(i, n) REP(j, m)#define REP_2_1(i, j, n, m) REP_1(i, n) REP_1(j, m)#define REP_3(i, j, k, n, m, l) REP(i, n) REP(j, m) REP(k, l)#define REP_3_1(i, j, k, n, m, l) REP_1(i, n) REP_1(j, m) REP_1(k, l)#define REP_4(i, j, k, ii, n, m, l, nn) REP(i, n) REP(j, m) REP(k, l) REP(ii, nn)#define REP_4_1(i, j, k, ii, n, m, l, nn) REP_1(i, n) REP_1(j, m) REP_1(k, l) REP_1(ii, nn)#define ALL(A) A.begin(), A.end()#define LLA(A) A.rbegin(), A.rend()#define CPY(A, B) memcpy(A, B, sizeof(A))#define INS(A, P, B) A.insert(A.begin() + P, B)#define ERS(A, P) A.erase(A.begin() + P)#define LBD(A, x) (lower_bound(ALL(A), x) - A.begin())#define UBD(A, x) (lower_bound(ALL(A), x) - A.begin())#define CTN(T, x) (T.find(x) != T.end())#define SZ(A) int((A).size())#define PB push_back#define MP(A, B) make_pair(A, B)#define PTT pair<T, T>#define Ts *this#define rTs return Ts#define fi first#define se second#define re real()#define im imag()#define Rush for(int ____T=RD(); ____T--;)#define Display(A, n, m) {                      \  REP(i, n){                                    \        REP(j, m-1) cout << A[i][j] << " ";     \        cout << A[i][m-1] << endl;                \    }                                            \}#define Display_1(A, n, m) {                    \    REP_1(i, n){                                \        REP_1(j, m-1) cout << A[i][j] << " ";   \        cout << A[i][m] << endl;                \    }                                            \}typedef long long LL;//typedef long double DB;typedef double DB;typedef unsigned uint;typedef unsigned long long uLL;typedef vector<int> VI;typedef vector<char> VC;typedef vector<string> VS;typedef vector<LL> VL;typedef vector<DB> VF;typedef set<int> SI;typedef set<string> SS;typedef map<int, int> MII;typedef map<string, int> MSI;typedef pair<int, int> PII;typedef pair<LL, LL> PLL;typedef vector<PII> VII;typedef vector<VI> VVI;typedef vector<VII> VVII;template<class T> inline T& RD(T &);template<class T> inline void OT(const T &);//inline int RD(){int x; return RD(x);}inline LL RD(){LL x; return RD(x);}inline DB& RF(DB &);inline DB RF(){DB x; return RF(x);}inline char* RS(char *s);inline char& RC(char &c);inline char RC();inline char& RC(char &c){scanf(" %c", &c); return c;}inline char RC(){char c; return RC(c);}//inline char& RC(char &c){c = getchar(); return c;}//inline char RC(){return getchar();}template<class T> inline T& RDD(T &);inline LL RDD(){LL x; return RDD(x);}template<class T0, class T1> inline T0& RD(T0 &x0, T1 &x1){RD(x0), RD(x1); return x0;}template<class T0, class T1, class T2> inline T0& RD(T0 &x0, T1 &x1, T2 &x2){RD(x0), RD(x1), RD(x2); return x0;}template<class T0, class T1, class T2, class T3> inline T0& RD(T0 &x0, T1 &x1, T2 &x2, T3 &x3){RD(x0), RD(x1), RD(x2), RD(x3); return x0;}template<class T0, class T1, class T2, class T3, class T4> inline T0& RD(T0 &x0, T1 &x1, T2 &x2, T3 &x3, T4 &x4){RD(x0), RD(x1), RD(x2), RD(x3), RD(x4); return x0;}template<class T0, class T1, class T2, class T3, class T4, class T5> inline T0& RD(T0 &x0, T1 &x1, T2 &x2, T3 &x3, T4 &x4, T5 &x5){RD(x0), RD(x1), RD(x2), RD(x3), RD(x4), RD(x5); return x0;}template<class T0, class T1, class T2, class T3, class T4, class T5, class T6> inline T0& RD(T0 &x0, T1 &x1, T2 &x2, T3 &x3, T4 &x4, T5 &x5, T6 &x6){RD(x0), RD(x1), RD(x2), RD(x3), RD(x4), RD(x5), RD(x6); return x0;}template<class T0, class T1> inline void OT(const T0 &x0, const T1 &x1){OT(x0), OT(x1);}template<class T0, class T1, class T2> inline void OT(const T0 &x0, const T1 &x1, const T2 &x2){OT(x0), OT(x1), OT(x2);}template<class T0, class T1, class T2, class T3> inline void OT(const T0 &x0, const T1 &x1, const T2 &x2, const T3 &x3){OT(x0), OT(x1), OT(x2), OT(x3);}template<class T0, class T1, class T2, class T3, class T4> inline void OT(const T0 &x0, const T1 &x1, const T2 &x2, const T3 &x3, const T4 &x4){OT(x0), OT(x1), OT(x2), OT(x3), OT(x4);}template<class T0, class T1, class T2, class T3, class T4, class T5> inline void OT(const T0 &x0, const T1 &x1, const T2 &x2, const T3 &x3, const T4 &x4, const T5 &x5){OT(x0), OT(x1), OT(x2), OT(x3), OT(x4), OT(x5);}template<class T0, class T1, class T2, class T3, class T4, class T5, class T6> inline void OT(const T0 &x0, const T1 &x1, const T2 &x2, const T3 &x3, const T4 &x4, const T5 &x5, const T6 &x6){OT(x0), OT(x1), OT(x2), OT(x3), OT(x4), OT(x5), OT(x6);}inline char& RC(char &a, char &b){RC(a), RC(b); return a;}inline char& RC(char &a, char &b, char &c){RC(a), RC(b), RC(c); return a;}inline char& RC(char &a, char &b, char &c, char &d){RC(a), RC(b), RC(c), RC(d); return a;}inline char& RC(char &a, char &b, char &c, char &d, char &e){RC(a), RC(b), RC(c), RC(d), RC(e); return a;}inline char& RC(char &a, char &b, char &c, char &d, char &e, char &f){RC(a), RC(b), RC(c), RC(d), RC(e), RC(f); return a;}inline char& RC(char &a, char &b, char &c, char &d, char &e, char &f, char &g){RC(a), RC(b), RC(c), RC(d), RC(e), RC(f), RC(g); return a;}inline DB& RF(DB &a, DB &b){RF(a), RF(b); return a;}inline DB& RF(DB &a, DB &b, DB &c){RF(a), RF(b), RF(c); return a;}inline DB& RF(DB &a, DB &b, DB &c, DB &d){RF(a), RF(b), RF(c), RF(d); return a;}inline DB& RF(DB &a, DB &b, DB &c, DB &d, DB &e){RF(a), RF(b), RF(c), RF(d), RF(e); return a;}inline DB& RF(DB &a, DB &b, DB &c, DB &d, DB &e, DB &f){RF(a), RF(b), RF(c), RF(d), RF(e), RF(f); return a;}inline DB& RF(DB &a, DB &b, DB &c, DB &d, DB &e, DB &f, DB &g){RF(a), RF(b), RF(c), RF(d), RF(e), RF(f), RF(g); return a;}inline void RS(char *s1, char *s2){RS(s1), RS(s2);}inline void RS(char *s1, char *s2, char *s3){RS(s1), RS(s2), RS(s3);}template<class T0,class T1>inline void RDD(T0&a, T1&b){RDD(a),RDD(b);}template<class T0,class T1,class T2>inline void RDD(T0&a, T1&b, T2&c){RDD(a),RDD(b),RDD(c);}template<class T> inline void RST(T &A){memset(A, 0, sizeof(A));}template<class T> inline void FLC(T &A, int x){memset(A, x, sizeof(A));}template<class T> inline void CLR(T &A){A.clear();}template<class T0, class T1> inline void RST(T0 &A0, T1 &A1){RST(A0), RST(A1);}template<class T0, class T1, class T2> inline void RST(T0 &A0, T1 &A1, T2 &A2){RST(A0), RST(A1), RST(A2);}template<class T0, class T1, class T2, class T3> inline void RST(T0 &A0, T1 &A1, T2 &A2, T3 &A3){RST(A0), RST(A1), RST(A2), RST(A3);}template<class T0, class T1, class T2, class T3, class T4> inline void RST(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4){RST(A0), RST(A1), RST(A2), RST(A3), RST(A4);}template<class T0, class T1, class T2, class T3, class T4, class T5> inline void RST(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, T5 &A5){RST(A0), RST(A1), RST(A2), RST(A3), RST(A4), RST(A5);}template<class T0, class T1, class T2, class T3, class T4, class T5, class T6> inline void RST(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, T5 &A5, T6 &A6){RST(A0), RST(A1), RST(A2), RST(A3), RST(A4), RST(A5), RST(A6);}template<class T0, class T1> inline void FLC(T0 &A0, T1 &A1, int x){FLC(A0, x), FLC(A1, x);}template<class T0, class T1, class T2> inline void FLC(T0 &A0, T1 &A1, T2 &A2, int x){FLC(A0, x), FLC(A1, x), FLC(A2, x);}template<class T0, class T1, class T2, class T3> inline void FLC(T0 &A0, T1 &A1, T2 &A2, T3 &A3, int x){FLC(A0, x), FLC(A1, x), FLC(A2, x), FLC(A3, x);}template<class T0, class T1, class T2, class T3, class T4> inline void FLC(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, int x){FLC(A0, x), FLC(A1, x), FLC(A2, x), FLC(A3, x), FLC(A4, x);}template<class T0, class T1, class T2, class T3, class T4, class T5> inline void FLC(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, T5 &A5, int x){FLC(A0, x), FLC(A1, x), FLC(A2, x), FLC(A3, x), FLC(A4, x), FLC(A5, x);}template<class T0, class T1, class T2, class T3, class T4, class T5, class T6> inline void FLC(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, T5 &A5, T6 &A6, int x){FLC(A0, x), FLC(A1, x), FLC(A2, x), FLC(A3, x), FLC(A4, x), FLC(A5, x), FLC(A6, x);}template<class T> inline void CLR(priority_queue<T, vector<T>, less<T> > &Q){while (!Q.empty()) Q.pop();}template<class T> inline void CLR(priority_queue<T, vector<T>, greater<T> > &Q){while (!Q.empty()) Q.pop();}template<class T> inline void CLR(stack<T> &S){while (!S.empty()) S.pop();}template<class T> inline void CLR(queue<T> &Q){while (!Q.empty()) Q.pop();}template<class T0, class T1> inline void CLR(T0 &A0, T1 &A1){CLR(A0), CLR(A1);}template<class T0, class T1, class T2> inline void CLR(T0 &A0, T1 &A1, T2 &A2){CLR(A0), CLR(A1), CLR(A2);}template<class T0, class T1, class T2, class T3> inline void CLR(T0 &A0, T1 &A1, T2 &A2, T3 &A3){CLR(A0), CLR(A1), CLR(A2), CLR(A3);}template<class T0, class T1, class T2, class T3, class T4> inline void CLR(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4){CLR(A0), CLR(A1), CLR(A2), CLR(A3), CLR(A4);}template<class T0, class T1, class T2, class T3, class T4, class T5> inline void CLR(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, T5 &A5){CLR(A0), CLR(A1), CLR(A2), CLR(A3), CLR(A4), CLR(A5);}template<class T0, class T1, class T2, class T3, class T4, class T5, class T6> inline void CLR(T0 &A0, T1 &A1, T2 &A2, T3 &A3, T4 &A4, T5 &A5, T6 &A6){CLR(A0), CLR(A1), CLR(A2), CLR(A3), CLR(A4), CLR(A5), CLR(A6);}template<class T> inline void CLR(T &A, int n){REP(i, n) CLR(A[i]);}template<class T> inline bool EPT(T &a){return a.empty();}template<class T> inline T& SRT(T &A){sort(ALL(A)); return A;}template<class T, class C> inline T& SRT(T &A, C B){sort(ALL(A), B); return A;}template<class T> inline T& RVS(T &A){reverse(ALL(A)); return A;}template<class T> inline T& UNQQ(T &A){A.resize(unique(ALL(A))-A.begin());return A;}template<class T> inline T& UNQ(T &A){SRT(A);return UNQQ(A);}//}/** Constant List .. **/ //{const int MOD = int(1e9)+7;const int INF = 0x3f3f3f3f;const LL INFF = 0x3f3f3f3f3f3f3f3fLL;const DB EPS = 1e-9;const DB OO = 1e20;const DB PI = acos(-1.0); //M_PI;//}/** Add On .. **/ //{// <<= '0. Nichi Joo ., //{template<class T> inline T& checkMin(T &a,const T b){if (b<a) a=b;return a;}template<class T> inline T& checkMax(T &a,const T b){if (a<b) a=b;return a;}template<class T> inline T& checkMin(T &a, T &b, const T x){checkMin(a, x), checkMin(b, x);return a;}template<class T> inline T& checkMax(T &a, T &b, const T x){checkMax(a, x), checkMax(b, x);return a;}template <class T, class C> inline T& checkMin(T& a, const T b, C c){if (c(b,a)) a = b;return a;}template <class T, class C> inline T& checkMax(T& a, const T b, C c){if (c(a,b)) a = b;return a;}template<class T> inline T min(T a, T b, T c){return min(min(a, b), c);}template<class T> inline T max(T a, T b, T c){return max(max(a, b), c);}template<class T> inline T min(T a, T b, T c, T d){return min(min(a, b), min(c, d));}template<class T> inline T max(T a, T b, T c, T d){return max(max(a, b), max(c, d));}template<class T> inline T min(T a, T b, T c, T d, T e){return min(min(min(a,b),min(c,d)),e);}template<class T> inline T max(T a, T b, T c, T d, T e){return max(max(max(a,b),max(c,d)),e);}template<class T> inline T sqr(T a){return a*a;}template<class T> inline T cub(T a){return a*a*a;}template<class T> inline T ceil(T x, T y){return (x - 1) / y + 1;}template<class T> T abs(T x){return x>0?x:-x;}inline int sgn(DB x){return x < -EPS ? -1 : x > EPS;}inline int sgn(DB x, DB y){return sgn(x - y);}inline DB cos(DB a, DB b, DB c){return (sqr(a)+sqr(b)-sqr(c))/(2*a*b);}inline DB cot(DB x){return 1./tan(x);};inline DB sec(DB x){return 1./cos(x);};inline DB csc(DB x){return 1./sin(x);};//}//基础包。。// <<= '2. Number Theory .,//{namespace NT{#define gcd __gcdinline LL lcm(LL a, LL b){return a*b/gcd(a,b);}inline void INC(int &a, int b){a += b; if (a >= MOD) a -= MOD;}inline int sum(int a, int b){a += b; if (a >= MOD) a -= MOD; return a;}/* 模数两倍刚好超 int 时。inline int sum(uint a, int b){a += b; a %= MOD;if (a < 0) a += MOD; return a;}inline void INC(int &a, int b){a = sum(a, b);}*/inline void DEC(int &a, int b){a -= b; if (a < 0) a += MOD;}inline int dff(int a, int b){a -= b; if (a < 0) a  += MOD; return a;}inline void MUL(int &a, int b){a = (LL)a * b % MOD;}inline int pdt(int a, int b){return (LL)a * b % MOD;}inline int gcd(int m, int n, int &x, int &y){    x = 1, y = 0; int xx = 0, yy = 1, q;    while (1){        q = m / n, m %= n;        if (!m){x = xx, y = yy; return n;}        DEC(x, pdt(q, xx)), DEC(y, pdt(q, yy));        q = n / m, n %= m;        if (!n) return m;        DEC(xx, pdt(q, x)), DEC(yy, pdt(q, y));    }}inline int sum(int a, int b, int c){return sum(a, sum(b, c));}inline int sum(int a, int b, int c, int d){return sum(sum(a, b), sum(c, d));}inline int pdt(int a, int b, int c){return pdt(a, pdt(b, c));}inline int pdt(int a, int b, int c, int d){return pdt(pdt(a, b), pdt(c, d));}inline int pow(int a, LL b){    int c(1); while (b){        if (b&1) MUL(c, a);        MUL(a, a), b >>= 1;    }    return c;}template<class T> inline T pow(T a, LL b){    T c(1); while (b){        if (b&1) c *= a;        a *= a, b >>= 1;    }    return c;}template<class T> inline T pow(T a, int b){    return pow(a, (LL)b);}inline int _I(int b){    int a = MOD, x1 = 0, x2 = 1, q; while (1){        q = a / b, a %= b;        if (!a) return x2;        DEC(x1, pdt(q, x2));        q = b / a, b %= a;        if (!b) return x1;        DEC(x2, pdt(q, x1));    }}inline void DIV(int &a, int b){MUL(a, _I(b));}inline int qtt(int a, int b){return pdt(a, _I(b));}} using namespace NT;//}//。。自带取模的环类。。struct Int{    int val;    operator int() const{return val;}    Int(int val = 0):val(val){        val %= MOD; if (val < 0) val += MOD;    }    Int(LL _val){        _val %= MOD; if (_val < 0) _val += MOD;        val = _val;    }    Int& operator +=(const int& rhs){INC(val, rhs);rTs;}    Int operator +(const int& rhs) const{return sum(val, rhs);}    Int& operator -=(const int& rhs){DEC(val, rhs);rTs;}    Int operator -(const int& rhs) const{return dff(val, rhs);}    Int& operator *=(const int& rhs){MUL(val, rhs);rTs;}    Int operator *(const int& rhs) const{return pdt(val, rhs);}    Int& operator /=(const int& rhs){DIV(val, rhs);rTs;}    Int operator /(const int& rhs) const{return qtt(val, rhs);}    Int operator-()const{return MOD-*this;}};//}/** I/O Accelerator Interface .. **/ //{#define g (c=getchar())#define d isdigit(g)#define p x=x*10+c-'0'#define n x=x*10+'0'-c#define pp l/=10,p#define nn l/=10,ntemplate<class T> inline T& RD(T &x){    char c;while(!d);x=c-'0';while(d)p;    return x;}template<class T> inline T& RDD(T &x){    char c;while(g,c!='-'&&!isdigit(c));    if (c=='-'){x='0'-g;while(d)n;}    else{x=c-'0';while(d)p;}    return x;}inline DB& RF(DB &x){    //scanf("%lf", &x);    char c;while(g,c!='-'&&c!='.'&&!isdigit(c));    if(c=='-')if(g=='.'){x=0;DB l=1;while(d)nn;x*=l;}        else{x='0'-c;while(d)n;if(c=='.'){DB l=1;while(d)nn;x*=l;}}    else if(c=='.'){x=0;DB l=1;while(d)pp;x*=l;}        else{x=c-'0';while(d)p;if(c=='.'){DB l=1;while(d)pp;x*=l;}}    return x;}#undef nn#undef pp#undef n#undef p#undef d#undef ginline char* RS(char *s){    //gets(s);    scanf("%s", s);    return s;}LL last_ans; int Case; template<class T> inline void OT(const T &x){    printf("Case #%d: ", ++Case);    //printf("%lld\n", x);    //printf("%.9f\n", x);    //printf("%d\n", x);    cout << x << endl;    //last_ans = x;}//}// <<= '1. Bitwise Operation ., //{namespace BO{inline bool _1(int x, int i){return bool(x&1<<i);}inline bool _1(LL x, int i){return bool(x&1LL<<i);}inline LL _1(int i){return 1LL<<i;}inline LL _U(int i){return _1(i) - 1;};inline int reverse_bits(int x){    x = ((x >> 1) & 0x55555555) | ((x << 1) & 0xaaaaaaaa);    x = ((x >> 2) & 0x33333333) | ((x << 2) & 0xcccccccc);    x = ((x >> 4) & 0x0f0f0f0f) | ((x << 4) & 0xf0f0f0f0);    x = ((x >> 8) & 0x00ff00ff) | ((x << 8) & 0xff00ff00);    x = ((x >>16) & 0x0000ffff) | ((x <<16) & 0xffff0000);    return x;}inline LL reverse_bits(LL x){    x = ((x >> 1) & 0x5555555555555555LL) | ((x << 1) & 0xaaaaaaaaaaaaaaaaLL);    x = ((x >> 2) & 0x3333333333333333LL) | ((x << 2) & 0xccccccccccccccccLL);    x = ((x >> 4) & 0x0f0f0f0f0f0f0f0fLL) | ((x << 4) & 0xf0f0f0f0f0f0f0f0LL);    x = ((x >> 8) & 0x00ff00ff00ff00ffLL) | ((x << 8) & 0xff00ff00ff00ff00LL);    x = ((x >>16) & 0x0000ffff0000ffffLL) | ((x <<16) & 0xffff0000ffff0000LL);    x = ((x >>32) & 0x00000000ffffffffLL) | ((x <<32) & 0xffffffff00000000LL);    return x;}template<class T> inline bool odd(T x){return x&1;}template<class T> inline bool even(T x){return !odd(x);}template<class T> inline T low_bit(T x) {return x & -x;}template<class T> inline T high_bit(T x) {T p = low_bit(x);while (p != x) x -= p, p = low_bit(x);return p;}template<class T> inline T cover_bit(T x){T p = 1; while (p < x) p <<= 1;return p;}template<class T> inline int cover_idx(T x){int p = 0; while (_1(p) < x ) ++p; return p;}inline int clz(int x){return __builtin_clz(x);}inline int clz(LL x){return __builtin_clzll(x);}inline int ctz(int x){return __builtin_ctz(x);}inline int ctz(LL x){return __builtin_ctzll(x);}inline int lg2(int x){return !x ? -1 : 31 - clz(x);}inline int lg2(LL x){return !x ? -1 : 63 - clz(x);}inline int low_idx(int x){return !x ? -1 : ctz(x);}inline int low_idx(LL x){return !x ? -1 : ctz(x);}inline int high_idx(int x){return lg2(x);}inline int high_idx(LL x){return lg2(x);}inline int parity(int x){return __builtin_parity(x);}inline int parity(LL x){return __builtin_parityll(x);}inline int count_bits(int x){return __builtin_popcount(x);}inline int count_bits(LL x){return __builtin_popcountll(x);}} using namespace BO;#define N 1000010int vis[N];int n , m , a, b;int main(){    #ifdef DoubleQ    freopen("in.txt","r",stdin);    #endif    while(~scanf("%d%d",&n,&m))    {        REP(i , n)        {            scanf("%d%d",&a,&b);            vis[a] += b;        }        int sum = 0;        FOR_1(i , 1 , N)        {            if(vis[i] > m)            {                sum += m;                vis[i] -= m;                if(vis[i] > m)                    vis[i+1] += m;                else                    vis[i+1] += vis[i];            }            else                sum += vis[i];        }        printf("%d\n",sum);    }}