#include "iostream"

#include "algorithm"

#include "cstdio"

using namespace std;

struct Point

{

    int x;

    int y;

    double z;

};

bool cmp(const Point &a,const Point &b)

{

    return a.z > b.z;

}

int main()

{

    while()

    {

        int M;

        int n;

        cin >> M >> n;

        if(M == - && n == -)

            break;

        Point *point = new Point[n];

        for(int i=;i<n;i++)                    //sort

        {

            cin >> point[i].x >> point[i].y ;

            point[i].z = double(point[i].x)/double(point[i].y);

        }

        sort(point,point+n,cmp);

        double food = ;

        int i;

        for(i=;i<n;i++)                   //caculate

        {

            if(M >= point[i].y)

            {

                M = M - point[i].y;

                food = food + point[i].x;

            }

            else

            {

                food = food + double(M) * point[i].x/point[i].y;     //当double与整数相乘(除)就会变成double，完美的解决了精度的问题。遇到精度问题时，一定要把高精度数放在前面。

                break;

            }

        }

        printf("%.3lf\n",food);

    }

    return ;

}