#include<bits/stdc++.h>
using namespace std;
int N, K;
double ans, dv,a[200];
void back(int poz, int k, double rez)
{
	if (k > K)
	{
		ans += rez;
		dv++;
		return;
	}
	for(int i = poz; i <= N-K+k; i++)
	{
		back(i+1,k+1,rez*a[i]);
	}
}
int main()
{
	ifstream cin("dezastru.in");
	ofstream cout("dezastru.out");
	cin >> N >> K;
	for (int i =  1; i <= N; i++) cin >> a[i];
	back(1,1,1);
	cout.precision(7);
	cout << fixed << (1.0*ans/dv);
}