#include <stdio.h>

#include <algorithm>

using namespace std;

struct Node

{

double j,f,p;

} node[10000];

int cmp(Node x,Node y)

{

return x.p>y.p;

}

int main()

{

int m,n;

while(~scanf("%d%d",&n,&m) && (m!=-1 || n!=-1))

{

double sum = 0,max = 0;

int i,j;

for(i = 0; i<m; i++)

{

scanf("%lf%lf",&node[i].j,&node[i].f);

node[i].p = node[i].j/node[i].f;

}

sort(node,node+m,cmp);

for(i = 0; i<m; i++)

{

if(n>node[i].f)

{

sum+=node[i].j;

n-=node[i].f;

}

else

{

sum+=node[i].p*n;

break;

}

}

printf("%.3lf\n",sum);

}

return 0;

}