#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
typedef long long ll;
using namespace std;
struct node
{
    int x,y;
    double num;
}a[10000];
bool cmp(node xx,node yy){
    return xx.num>yy.num;
}
int main(){
    int m,n;
    int i,j;
    while(scanf("%d%d",&m,&n)!=EOF){
        if(m==-1&&n==-1)
            break;
        for(i=1;i<=n;i++){
            scanf("%d%d",&a[i].x,&a[i].y);
            a[i].num=(double)a[i].x/a[i].y;
        }
        sort(a+1,a+n+1,cmp);
        double sum=0;
        for(i=1;i<=n;i++){
            //cout<<a[i].num<<endl;
            if(a[i].y<=m)
            {
                sum=sum+a[i].x;
                m=m-a[i].y;
            }
            else{
                sum=sum+a[i].num*m;
                break;
            }
        }
        printf("%.3f\n",sum);
    }
}