#include <string.h>
#include <iostream>
#include <algorithm>
#include <stdlib.h>
#include <stdio.h>
using namespace std;
struct sa
{
    int a,b;
    double c;
}data[3005];
bool cmp(sa x,sa y)
{
    return x.c>y.c;
}
int main()
{
    int i,j,m,n;
    double sum;
    while (cin>>m>>n&&m!=-1&&n!=-1)
    {
        sum=0;
        for (i=0;i<n;i++)
        {
            cin>>data[i].a>>data[i].b;
            data[i].c=(double)data[i].a/(double)data[i].b;
        }
        sort(data,data+n,cmp);
        //for (i=0;i<n;i++)
        //cout<<data[i].a<<" "<<data[i].b<<" "<<data[i].c<<endl;
        for (i=0;i<n;i++)
        {
            if (m>=data[i].b)
            {
                m-=data[i].b;
                sum+=data[i].a;
            }
            else if (m<data[i].b&&m>0)
            {
                sum+=data[i].c*(double)m;
                m=0;
            }
            else
            break;
        }
        printf("%.3lf\n",sum);
    }
    return 0;
}