#include <iostream>
#include <stdio.h>
#include <algorithm>

using namespace std;

struct dat
{
    int j;
    int f;
    double sc;
} data[1000];

bool cmp(dat a, dat b)
{
    return a.sc>b.sc;
}

int main()
{
    int m,n;
    double ans;
    while(scanf("%d%d",&m,&n) && m!=-1 && n!=-1)
    {
        for(int i=0; i<n; i++)
        {
            scanf("%d%d",&data[i].j, &data[i].f);
            data[i].sc = (double)data[i].j/(double)data[i].f;
        }

        sort(data, data+n, cmp);

        ans = 0;
        for(int i=0; i<n; i++)
        {
            if(data[i].f<=m)
            {
                ans+=data[i].j;
                m-=data[i].f;
            }
            else
            {
                ans+=data[i].sc*(double)m;
                break;
            }

        }
        printf("%.3f\n", ans);
    }

    return 0;
}