CodeForces 16B Burglar and Matches (贪心)

题意   一个窃贼到火柴仓库偷火柴   仓库有m个容器    第i个容器有a[i]个火柴盒  其中每个火柴盒中有b[i]根火柴   窃贼最大可以拿n个火柴盒  输入n m  然后m行a[i]  b[i]   求窃贼最多能偷多少根火柴

很水的贪心  直接每次选当前火柴最多的盒子  n减去盒子数  直到n=0 

#include<cstdio>  #include<cstring>  #include<algorithm>  using namespace std;  const int M = 25;  int a[M], b[M], c[M], n, m, ans;    bool cmp (int i, int j)  {      return b[i] > b[j];  }    int main()  {      scanf ("%d%d", &n, &m);      for (int i = 1; i <= m; ++i)      {          c[i] = i;          scanf ("%d%d", &a[i], &b[i]);      }      sort (c + 1, c + m + 1, cmp);      ans = 0;      for (int i = 1; i <= m; ++i)      {          if (n >= a[c[i]])          {              n -= a[c[i]];              ans += a[c[i]] * b[c[i]];          }          else          {              ans += n * b[c[i]];              break;          }      }      printf ("%d\n", ans);      return 0;  }  

A burglar got into a matches warehouse and wants to steal as many matches as possible. In the warehouse there are mcontainers, in the i-th container there are ai matchboxes, and each matchbox contains bi matches. All the matchboxes are of the same size. The burglar's rucksack can hold n matchboxes exactly. Your task is to find out the maximum amount of matches that a burglar can carry away. He has no time to rearrange matches in the matchboxes, that's why he just chooses not more than n matchboxes so that the total amount of matches in them is maximal.

Input

The first line of the input contains integer n (1 ≤ n ≤ 2·108) and integer m (1 ≤ m ≤ 20). The i + 1-th line contains a pair of numbers ai and bi (1 ≤ ai ≤ 108, 1 ≤ bi ≤ 10). All the input numbers are integer.

Output

Output the only number — answer to the problem.

Sample test(s)

input

7 35 102 53 6

output

62

input

3 31 32 23 1

output

7