#include <bits/stdc++.h>
using namespace std;

 typedef struct question {
    double w;//题数
    double m;//体力
}q;

bool cmp(question a, question b) {
    return a.m/a.w<b.m/b.w;
}
int main() {
   int total,n;
    while(cin>>total>>n) {
        if(total==-1||n==-1) {
            break;
        }
        vector<question> v;
        for(int i=0;i<n;i++) {
            double x,y;
            cin>>x>>y;
            v.push_back({x,y});
        }
        double ans=0;
        sort(v.begin(),v.end(),cmp);
        for(int i=0;i<n;i++) {
            if (total>=v[i].m) {
                total-=v[i].m;
                ans+=v[i].w;
            }
            else {
                ans+=total/v[i].m*v[i].w;
                break;
            }
        }
        printf("%.3lf\n",ans);
    }
}