#include<iostream>
#include<fstream>
#include<algorithm>
#include<iomanip>
using namespace std;
float s,p;
unsigned long long int fact;
int main(void)
{
	int n,k,i;
	float a[30];
	fstream f("dezastru.in",ios::in),g("dezastru.out",ios::out);
	f>>n>>k;
	for (i=1;i<=n;i++)
		f>>a[i];
	fact=1;
	for (i=1;i<=n;i++)
		fact*=i;
	do
	{
		p=1;
		for(i=1;i<=k;i++)
			p=p*a[i];
		s=s+p;
	}
	while (next_permutation(a+1,a+n+1));
	//cout<<fact;
	g<<fixed<<setprecision(6)<<double(s/fact);
}