【codeforces 546A】Soldier and Bananas

time limit per test1 second
memory limit per test256 megabytes
inputstandard input
outputstandard output
A soldier wants to buy w bananas in the shop. He has to pay k dollars for the first banana, 2k dollars for the second one and so on (in other words, he has to pay i·k dollars for the i-th banana).

He has n dollars. How many dollars does he have to borrow from his friend soldier to buy w bananas?

Input
The first line contains three positive integers k, n, w (1  ≤  k, w  ≤  1000, 0 ≤ n ≤ 109), the cost of the first banana, initial number of dollars the soldier has and number of bananas he wants.

Output
Output one integer — the amount of dollars that the soldier must borrow from his friend. If he doesn’t have to borrow money, output 0.

Examples
input
3 17 4
output
13

【题目链接】:http://codeforces.com/contest/546/problem/A

【题解】

就是w个数的等差数列的求和公式。
然后如果钱够的话输出的是0！！！！
否则输出差的绝对值就好.
细心。

【完整代码】

#include <bits/stdc++.h>using namespace std;#define lson l,m,rt<<1#define rson m+1,r,rt<<1|1#define LL long long#define rep1(i,a,b) for (int i = a;i <= b;i++)#define rep2(i,a,b) for (int i = a;i >= b;i--)#define mp make_pair#define pb push_back#define fi first#define se second#define rei(x) scanf("%d",&x)#define rel(x) scanf("%I64d",&x)typedef pair<int,int> pii;typedef pair<LL,LL> pll;//const int MAXN = x;const int dx[9] = {0,1,-1,0,0,-1,-1,1,1};const int dy[9] = {0,0,0,-1,1,-1,1,-1,1};const double pi = acos(-1.0);LL k,n,w;int main(){    //freopen("F:\\rush.txt","r",stdin);    rel(k);rel(n);rel(w);    LL temp1 = (k + w*k)*w/2;    LL temp = n-temp1;    if (temp <0)        cout << abs(temp)<<endl;    else        puts("0");    return 0;}