HDU 1009 FatMouse' Trade

#include <bits/stdc++.h>
using namespace std;

const int maxn = 1e5 + 10;
int N, M;

struct Node{
    double j;
    double f;
    double pri;
}node[maxn];

bool cmp(const Node& a, const Node& b) {
    return a.pri > b.pri;
}

int main() {
    while(~scanf("%d%d", &M, &N)) {
        if(N == -1 && M == -1) break;
        for(int i = 1; i <= N; i ++) {
            scanf("%lf%lf", &node[i].j, &node[i].f);
            node[i].pri = node[i].j / node[i].f;
        }

        double num = 0.0;
        sort(node + 1, node + 1 + N, cmp);
        for(int i = 1; i <= N; i ++) {
            if(M >= node[i].f) {
                num += node[i].j;
                M -= node[i].f;
            }
            else {
                num += M * node[i].j / node[i].f;
                break;
            }
        }
        printf("%.3lf\n", num);
    }
    return 0;
}