HDU 1009

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

struct sg{
    double javabean;
    double foodcat;
    double price;
};


struct sg input[1005];

int cmp(const void *a, const void *b)
{
   
    return (*(struct sg *)a).price >  (*(struct sg *)b).price ? 1 : -1;
}

int main(void)
{
    int i;
    int M, N;
    double sum;
    double sur;
    while(scanf("%d%d", &M, &N) && (M != -1 && N != -1)) {
        for(i = 0; i < N; i++)    {
            scanf("%lf%lf", &input[i].javabean, &input[i].foodcat);    
            input[i].price = (input[i].foodcat) / input[i].javabean;
        }
            
        sum = 0;
        sur = M;

        qsort(input, N, sizeof(struct sg), cmp); 

        for(i = 0; i < N; i++) {
            double tmp = input[i].foodcat;
            if(tmp < sur && sur > 0)  {
                sum += input[i].javabean;
                sur -= input[i].foodcat;
            } else if(sur > 0) {
                sum += (sur / input[i].foodcat * input[i].javabean);
                sur = 0.0;
                break;
            }    
        
        }
        printf("%0.3lf\n", sum);    
    
    }
    
    return 0;
}