https://vjudge.net/contest/279730#problem/K
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<algorithm>
using namespace std;
struct str
{
int x,y;
double z;
bool operator<(const str &str1)const
{
if (z>str1.z) return 1;else return 0;
}
} a[100000];
int i,n,m,x,sum,y,j,f,b[100000],c[100000];
double ans;
int main()
{
while (~scanf("%d%d",&m,&n))
{
if (m==-1&&n==-1) break;
for (i=1;i<=n; i++)
{
scanf("%d%d",&a[i].x,&a[i].y);
a[i].z=a[i].x*1.0/a[i].y;
}
sort(a+1,a+1+n);
sum=0;
ans=0;
for (i=1; i<=n; i++)
if (sum<=m)
{
if (sum+a[i].y<=m)
{
ans+=a[i].x;
sum+=a[i].y;
}
else
{
ans+=a[i].z*(m-sum);
sum=m;
}
}else break;
printf("%.3lf\n",ans);
}
}