include<stdio.h>

include

include<string.h>

using namespace std;

struct List
{
int java;
int cat;
float math;
}list[1000];

bool cmp(List a,List b)
{
if(a.math>b.math)
{
return 1;
}
return 0;
}

int main()
{
int m,n;
while(scanf("%d %d",&m,&n)!=EOF)
{
if(m==-1&&n==-1)return 0;
for(int i=0;i<n;i++)
{
scanf("%d %d",&list[i].java,&list[i].cat);
list[i].math=(float)list[i].java/(float)list[i].cat;
}
sort(list,list+n,cmp);
double sum=0;
for(int i=0;i<n;i++)
{
if(list[i].cat>m)
{
sum=sum+m*list[i].math;
break;
}
{
sum=sum+list[i].java;
m=m-list[i].cat;
}
}
printf("%.3lf\n",sum);
}
return 0;
}