P1208 混合牛奶

#include<iostream>
#include<cstring>
#include<string>
#include<algorithm>
#include<queue>
#include<vector>
#include<cstdio> 
using namespace std;
int n,m,ans=0;
struct H{
    int p,a;
}milk[5009];
int my_comp(const H&x,const H&y)
{
    if(x.p<y.p) return 1;
    return 0;
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&milk[i].p,&milk[i].a);
    }
    sort(milk+1,milk+m+1,my_comp);

    for(int i=1;i<=m;i++)
    {
        if(milk[i].a<=n)
        {
            ans+=milk[i].p*milk[i].a;
            n-=milk[i].a;
        }
        else 
        ans+=n*milk[i].p,n=0;

        if(n==0) break;
    }
    printf("%d",ans);
    return 0;
}