【hdu - 1009】

#include<iostream>
#include<iomanip>
using namespace std;
double a[1000];
double b[1000];
double c[1000];
int main()
{

    int m,n,i,j;
    while((cin >> m >> n) && (m != -1 && n != -1))
    {
        double sum = 0.0;
        for( i = 0; i < n; i++) { cin >> a[i] >> b[i]; c[i] = a[i]/b[i]; }
        for( i = 0; i < n; i++)
            for( j = 0; j < n-i-1; j++)
            {
                if(c[j] < c[j+1])
                {
                    double temp;
                    temp = a[j]; a[j] = a[j+1]; a[j+1] = temp;
                    temp = b[j]; b[j] = b[j+1]; b[j+1] = temp;
                    temp = c[j]; c[j] = c[j+1]; c[j+1] = temp;
                }
            }
        for( i = 0; i < n; i++)
        {
            if (m >= b[i]) { sum += a[i]; m -= b[i]; }
            else { sum += a[i] * (double)m / b[i]; break; }
        }
        cout << setiosflags(ios::fixed) << setprecision(3) << sum << endl;
    }
    return 0;
}