ZOJ 2109 FatMouse' Trade

/*
抄书上的程序..
用c语言写的老是WA
每次选效益最高的交换
*/
#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
using namespace std;
struct Mouse
{
    double j,f;
    double shouyi;
};
bool comp(const Mouse &d1,const Mouse &d2){
    if(d1.shouyi != d2.shouyi)
        return d2.shouyi < d1.shouyi;
    return d2.f > d1.f;
}
int main(){
    //ifstream cin("2109.txt");
    vector<Mouse> v;
    Mouse mouse;
    int m,n,i;
    cout.precision(3); //三位小数
    double sum;
    while(cin >> m >> n){
        if(m == -1 && n == -1)
            break;
        v.clear();
        sum = 0.0;
        for(i = 0; i < n; ++i){
            cin >> mouse.j >> mouse.f;
            mouse.shouyi = mouse.j / mouse.f;
            v.push_back(mouse);
        }
        sort(v.begin(),v.end(),comp);
        for(i = 0; i < v.size(); i++){
            if(m >= v[i].f){
                sum += v[i].j;
                m -= (int)v[i].f;
            } else {
                sum += m * v[i].shouyi;
                break;
            }
        }
        cout << fixed << sum << endl;
    }
    return 0;
}