#include <iostream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <vector>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int n, k;
long long nrp, b[26];
long double prod, prob[26];

void read()
{
	f >> n >> k;
	for(int i = 1; i <= n; ++i)
		f >> prob[i];
	f.close();
}

void back(int niv)
{
	if(niv == k+1)
	{
		double aux = 1;
		prod += aux;
		nrp++;
		return;
	}
	for(int i = b[niv-1] + 1; i <= n; ++i)
	{
		b[niv] = i;
		back(niv + 1);
	}
}

void solve()
{		
	nrp = 0;
	prod = 0;

	back(1);
	
	g << fixed << setprecision(6) << (double)prod / nrp;
	/*do {
		prod += prob[nrp/k];
		
		for(int i = 0; i < perm.size(); ++i)
			cout << perm[i] << " ";
		cout << '\n';
		nrp++;
	} while(next_permutation(perm.begin(), perm.end()));*/
		
}

int main()
{
	read();
	
	solve();
	
	return 0;
}