USACO 1.3.1

简单的贪心,将cent从小到大排序.
/*
ID:wang9621
PROG:milk
LANG:C++
*/
#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
struct edge
{
    int cent;
    int unit;
};
edge per[5005];
bool cmp(edge a,edge b)
{
    return a.cent<b.cent;
}
int main()
{
    freopen("milk.in","r",stdin);
    freopen("milk.out","w",stdout);
    int n,m;
    scanf("%d %d",&n,&m);
    for(int i = 1; i<=m; i++)
    {
        scanf("%d %d",&per[i].cent,&per[i].unit);
    }
    sort(per+1,per+m+1,cmp);
    int sum = 0;
    for(int i = 1; i<=m; i++)
    {
        if(n>=per[i].unit)
        {
            sum += per[i].cent*per[i].unit;
            n -= per[i].unit;
        }
        else if(n>0)
        {
            sum += n*per[i].cent;
            n = 0;
        }

    }
    printf("%d
",sum);
    return 0;
}