#include <iostream.h>
#include <stdlib.h>
#include <stdio.h>
#include <fstream.h>
ifstream f("mouse.in");
#define cin f
int comp(const void *m,const void *n)
{
return ((int *)n)[0]/((int *)n)[1]-((int *)m)[0]/((int *)m)[1];
}
    

int main()
{
    int m,n;
    int a[1000][2];
    int i,j;
    while(cin>>m>>n)
    {
        if (m==-1 && n==-1)
        return 0;
        for(i=0;i<n;i++)
        cin>>a[i][0]>>a[i][1];
        qsort(a,n,2*sizeof(int),comp);
        float k=(float)m;
        i=0;
        float Msum=0.0;
        //for(i=0;i<n;i++)
        //cout<<a[i][0]<<" "<<a[i][1]<<endl;
        //i=0;
        while(1)
        {
            if(k-(float)a[i][1]>0.00001)
            {
            Msum=Msum+a[i][0];
            //i++;
            k=k-(float)a[i][1];
            i++;
            }    
            else
            {
                Msum=Msum+k*((float)((float)a[i][0]/a[i][1]));
                break;
            }
        }
        //cout<<Msum<<endl;    
        printf("%0.3f\n",Msum);    
        
    }   
} 

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