#include<bits/stdc++.h>
using namespace std;
struct bankuai{
	double a;
	double b;
	double c=b/a;
}aa[1005];
 
bool cmp(bankuai a1,bankuai a2){
	return a1.c<a2.c;
}
int main(){	
	int m,n;
	while(cin>>m>>n){
		if(n==-1&&m==-1) break;		
		for(int i=0;i<n;i++){
			cin>>aa[i].a>>aa[i].b;			
		}
		sort(aa,aa+n,cmp);		
		double sum=0;
		for(int i=0;i<n;i++){
			if(m>=aa[i].b){
				sum+=aa[i].a;
				m-=aa[i].b;
			}
			else{
				sum+=(m/aa[i].c);
				break;
			}
		}		
		cout<<fixed<<setprecision(3)<<sum<<endl;
	}
}