【USACO1.3】 混合牛奶 Mixing Milk

#include<bits/stdc++.h>
using namespace std;
int n,m,ans;
struct node{ //结构体暴力破解
    int s,w;
    bool operator < (const node &o) const{
        if(s!=o.s) return s<o.s;
        return w>o.w;
    }
}a[5005];
int main()
{
    cin>>n>>m;
    for(int i=1;i<=m;i++) cin>>a[i].s>>a[i].w;
    sort(a+1,a+1+m);
    int i=1,tmp=n; //man
    while(tmp){ //what
        if(a[i].w!=0) //can
        {
            a[i].w--; //I
            ans+=a[i].s;
            tmp--; //say
        }
        else i++; //曼巴
    }
    cout<<ans; //out
    return 0;
}