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#include<stdio.h>
#include<algorithm>
using namespace std;
struct node {
    int j,f;
    double m;
} a[1200];
bool cmp(node A,node B) {
    return A.m>B.m;
}
int main() {
    int m,n;
    while(scanf("%d %d",&m,&n),m!=-1||n!=-1) {
        double sum=0;
        for(int i=0; i<n; i++) {
            scanf("%d %d",&a[i].j,&a[i].f);
            if(a[i].f>0)
                a[i].m=(double)a[i].j/a[i].f;
            else {
                a[i].m=0;
                sum+=a[i].j;
            }
        }
        sort(a,a+n,cmp);
        for(int i=0; i<n; i++) {
            sum+=min(m,a[i].f)*a[i].m;
            m-=min(m,a[i].f);
        }
        printf("%.3lf\n",sum);
    }
    return 0;
}