/** \brief hdu 1009--greedy
*
* \param date 2014/7/18
* \param state AC
* \return
*
*/
#include <iostream>
#include <fstream>
#include <algorithm>
#include <cstring>
using namespace std;
const int MAXN=1001;
struct Data
{
int J;
int F;
/*
bool operator < (const Data& d) const
{
return d.
}
*/
double price;
};
Data data[MAXN];
bool Comp(const Data& a,const Data& b)
{
if(a.price>=b.price)
return true;
else return false;
}
//void Knapsack(int n,int m,int v[],int w[],int x[])
void Knapsack(int n,int m)
{
//Sort(data,data+n,Comp);
sort(data,data+n,Comp);
int i;
int c=m;
double sum=0.0;
for(i=0;i<n;i++)
{
if(c>=data[i].F)
{
c-=data[i].F;
sum+=data[i].J;
}
else
{
sum+=c*1.0/data[i].F*data[i].J;
c=0;
}
}
printf("%.3f",sum);
cout<<endl;
}
int main()
{
//freopen("input.txt","r",stdin);
int M,N;
while(scanf("%d%d",&M,&N)!=EOF)
{
if(M==-1 && N==-1)break;
for(int i=0;i<N;i++)
{
//cin>>J[i]>>F[i];
cin>>data[i].J>>data[i].F;
data[i].price=data[i].J*1.0/data[i].F;
}
//
Knapsack(N,M);
}
return 0;
}