#include<cstdio>
#include<algorithm>
using namespace std;
struct Huan
{
    double j,f;
    double c;
}huan[3005];
bool cmp( Huan a, Huan b)
{
        return a.c > b.c ;
}
int main()
{
    double m;
    int n,t;
    while( scanf("%lf%d", &m, &n)!=EOF && ( m!=-1 || n!=-1 ) )
    {
        int i, j, k;
        double sum=0; 
        for( i=0; i<n; i++)
        {
            scanf("%lf%lf", &huan[i].j, &huan[i].f);
            huan[i].c = huan[i].j / huan[i].f;        }
        sort(huan, huan+n, cmp);//按f从小到大排序 
        for( i=0; i<n; i++)
        {
            if( m >= huan[i].f )//全盘换了 
            {    sum += huan[i].j;    m -=  huan[i].f;    }   
            else//大米的数量不足全盘换 
            {   sum +=  m*huan[i].c ;    break;    }      
        }
        printf("%.3lf\n",sum);
    }
    return 0;
}