HDU1009 FatMouse' Trade(贪心)

#include <iostream>
#include <vector>
#include <algorithm>
 
using namespace std;
 
struct node{
    double j,f;
    double pr;
}food[10100];
 
cmp cmp(node a,node b){
    if(a.pr == b.pr) return a.f < b.f;
    return a.pr > b.pr;
}
 
int main(){
    int n,m;
    while(scanf("%d%d",&m,&n) && n != -1 && m != -1){
        for( int i = 0;i < n;i ++){
            scanf("%lf%lf",&food[i].j,&food[i].f);
            food[i].pr = food[i].j / food[i].f ;
        }
        sort(food,food + n,cmp);
        double sum = 0;
        for(int i = 0;i < n;i ++){
            if(m <= food[i].f ){
                sum += food[i].j * (m / food[i].f);
                break;
            }
            if(m < 0){
                break;
            }
            sum += food[i].j;
            m -= food[i].f;
        }
        printf("%.3lf\n",sum);
    }
    
    return 0;
}