hdu 1009 FatMouse' Trade (Greedy)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <string>
#include <algorithm>

using namespace std;
const int N = 1005;
struct node {
    int J;
    int F;
}t[N];
bool cmp(const node a, const node b) {
    return double(a.J)/double(a.F) > double(b.J)/double(b.F);
}
int n, m;
int main() {
    int i;
    while(~scanf("%d%d", &m, &n)) {
        if(n == -1 && m == -1) break;
        for(i = 0; i < n; i++) {
            scanf("%d%d", &t[i].J, &t[i].F);
        }
        sort(t, t+n, cmp);
        double sum = 0;
        for(i = 0; i < n; i++) {
            if(m >= t[i].F) {
                m -= t[i].F;
                sum += t[i].J;
            }
            else {
                sum += (double)(t[i].J*m) / t[i].F;
                m = 0;
            }
            if(m == 0) break;
        }
        printf("%.3lf\n", sum);
    }
    return 0;
}