[2016-02-04][HDU][1009][FatMouse' Trade]

#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn = 1000 + 10;
struct room{ 
    int j,f;
    bool operator < (const room & a)const {
        return ((double)j / f ) > ((double)a.j / a.f ) ;
    }
}r[maxn];

int main(){
    int n,m;
    while(~scanf("%d%d",&m,&n) &&(n != -1 && m != -1)){
        for(int i = 0;i < n;i++){
            scanf("%d%d",&r[i].j,&r[i].f);
        }
        sort(r,r+n);
        double res = 0,left = m;
        for(int i = 0; i < n ;i++){
            if(left >= r[i].f){
                left -= r[i].f;
                res += r[i].j;
            }else {
                res += (left / r[i].f) * r[i].j;
                break;
            }    
        }    
        printf("%.3f\n",res);
    }
    return 0;
}