5 3
7 2
4 3
5 2
20 3
25 18
24 15
15 10
-1 -1

13.333
31.500

#include<iostream>
#include<algorithm>
using namespace std;
struct room
{
    int catfood;
    int mousefood;
    double danwei;
};
int cmp(room a,room b)
{
       return a.danwei>b.danwei;
}
int main()
{
    int M,N,i;
    room a[1000];
    while(scanf("%d%d",&M,&N))
    {
        double sum=0;
        if(M==-1 && N==-1)
            break;
        for(i=0;i<N;i++)
        {
            cin>>a[i].mousefood>>a[i].catfood;
        }
        for(i=0;i<N;i++)
           a[i].danwei=a[i].mousefood*1.0/a[i].catfood;//单位猫食，获得的物品量
        sort(a,a+N,cmp);//降序排列
        i=0;
        while(i<N && a[i].catfood<=M)
        {
            sum=sum+a[i].mousefood;
            M=M-a[i].catfood;
            i++;
        }
        if(i==N)
        {
            printf("%.3lf\n",sum);//准备的猫食可能有剩余
        }
        else
        {
            sum=sum+a[i].mousefood*M*1.0/a[i].catfood;//此时准备的猫食以花干
            printf("%.3lf\n",sum);
        }    
    }
    return 0;
}