#include
     
      #include
      
       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;}