FatMouse'Trade

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

struct door {
    double w;
    double p;
    bool operator < (const door & a) {
        return w/p>a.w/a.p;
    }
};

int main() {
    int m,n;
    while(cin>>m>>n) {
        if(m==-1&&n==-1) {
            break;
        }
        door a[n];
        for(int i=0; i<n; i++) {
            cin>>a[i].w>>a[i].p;
        }
        sort(a,a+n);
        double answer=0;
        for(int i=0; i<n; i++) {
            if(m>=a[i].p) {
                answer+=a[i].w;
                m-=a[i].p;
            } else {
                answer+=a[i].w/a[i].p*m;
                break;
            }
        }
        printf("%.3f\n",answer);
    }
    return 0;
}