#include <bits/stdc++.h>
using namespace std;

int n, k, r, p[26];
double a[26], b[26], sum;

ofstream fout("dezastru.out");

void back(int lvl)
{
	if (lvl == k + 1)
	{
		double prod = 1;
		for (int i = 1; i <= k; i++)
			prod *= a[i];
		sum += prod;
		r++;
		return;
	}
	for (int i = p[lvl - 1] + 1; i <= n; i++)
	{
		p[lvl] = i;
		a[lvl] = b[i];
		back(lvl + 1);
	}
}

int main()
{
	ifstream cin("dezastru.in");
	cin >> n >> k;
	for (int i = 1; i <= n; i++)
		cin >> b[i];
	sort(b + 1, b + n + 1);
	a[0] = 0;
	p[0] = 0;
	back(1);
	fout << sum / r;
}