hdu 1009(贪心)

//贪心，比例从大往后贪
#include <cstdio>
#include <cstdlib>
#include <iostream>

using namespace std;

struct node {
    double j;
    double f;
    double c;
}s[100000];

int cmp(const void *a, const void *b) {
    node *c = (node *)a;
    node *d = (node *)b;
    if (c->c > d->c) return -1;
    return 1;
}

int main() {
    int m, n;
    while (scanf("%d%d", &m, &n) && (m != -1 || n != -1)) {
        int i;
        for (i=0; i<n; ++i) {
            scanf ("%lf%lf", &s[i].j, &s[i].f);
            s[i].c = s[i].j / s[i].f;  //计算比例
        }
        qsort(s, n, sizeof(s[0]), cmp);
        double sum = 0;
        for (i=0; i<n; ++i) {  //贪心
            if (m >= s[i].f) {
                sum += s[i].j;
                m -= s[i].f;
            }
            else {
                sum += m / s[i].f * s[i].j;
                break;
            }
        }
        printf ("%.3lf\n", sum);
    }
    return 0;
}