#include<iostream>
#include<stdio.h>
#include<algorithm>
using namespace std;
struct Node
{
    double f,j;
    double wei;
}room[100005];
bool cmp(Node a,Node b)
{
    return a.wei>b.wei;
}
int main()
{
    double m;
    int n;
    while(scanf("%lf%d",&m,&n)&&(m!=-1&&n!=-1))
    {
        for(int i=0;i<n;i++)
        {
            scanf("%lf%lf",&room[i].j,&room[i].f);
            room[i].wei=room[i].j/room[i].f;
        }
        sort(room,room+n,cmp);
        double ans=0;
        for(int i=0;i<n;i++)
        {
            if(m<=0) break;
            if(m>=room[i].f) {m-=room[i].f;ans+=room[i].j;}
            else if(m<room[i].f){ans+=m*room[i].wei;m-=m;break;}
        }
        printf("%.3lf\n",ans);
    }
}