#include<cstdio>
#include<vector>
#include<algorithm>
using namespace std;
int main()
{
	vector<float> a;
	int i,j,n,m,k,p=0;
	float x,s=0;
freopen("dezastru.in","r",stdin);
//freopen("dezastru.out","w",stdout);
scanf("%d%d",&n,&k);
for(i=1;i<=n;i++)
{
	scanf("%f",&x);
	a.push_back(x);
}
sort(a.begin(),a.end());
do
{
	x=1; p++;
	for(i=0;i<k;i++)
	{
		x*=a[i];
	}
	s+=x;
	//printf("%f %f\n",x,s);
}
while(next_permutation(a.begin(),a.end()));
s=s/p;
printf("%f",s);
return 0;
}