hdu1009

#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;
}