09 P1208 [USACO1.3] 混合牛奶 Mixing Milk

题目：

代码：

#include<iostream>
using namespace std;


#include<algorithm>

struct far
{
    double pri;
    double num;
}sa[10005];

bool cmp(far f1,far f2)
{
    return f1.pri<f2.pri;
}

int main()
{
    int n;cin>>n;
    int m;cin>>m;
    for(int i=1;i<=m;i++)
    {
        cin>>sa[i].pri>>sa[i].num;
    }
    
    sort(sa+1,sa+m+1,cmp);
    
    
    double sum_n=0,sum_p=0;
    for(int i=1;i<=m;i++)
    {
        if(sum_n+sa[i].num<=n)
        {
            sum_p+=sa[i].num*sa[i].pri;
            sum_n+=sa[i].num;
        }
        else
        {
            sum_p+=(n-sum_n)*sa[i].pri;
            break;
        }
    }
    
    cout<<sum_p<<endl;
}