语言：C++14 (GCC 9)

#include <bits/stdc++.h>
using namespace std;
int n,m;
struct milk
{
    int pi,ai;
}mk[10000];
bool cmp(milk a,milk b)
{
    return a.pi<b.pi;
}
int main()
{
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++)
    {
        scanf("%d%d",&mk[i].pi,&mk[i].ai);
    }
    sort(mk,mk+m,cmp);
    int i=0,mon=0;
    while(1)
    {
        if(n>=mk[i].ai)
        {
            mon+=mk[i].ai*mk[i].pi;
            n-=mk[i].ai;
        }
        else
        {
            mon+=mk[i].pi*n;
            break;
        }
        i++;
    }
    printf("%d",mon);
    return 0;
}

两个RE 拜托了！