#include <iostream>
#include <fstream>
#include <queue>
using namespace std;

#define DIM 30

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");


int N, K;
int x[DIM];
double p[DIM];
bool s[DIM];

double Sol = 0.0;
double aux = 1.0;
int Nr = 1;
int index;

void Read();
double Fact(int x);
void Back(int k);
void Write(int k);

int main()
{
	Read();

	for (int i = N - K + 1; i <= N; ++i)
		Nr *= i;


	Back(1);

	fout << Sol;

	fin.close();
	fout.close();
	return 0;
}

void Back(int k)
{
	if (k > K)
	{
		Sol += 2 * (aux / (double)Nr);
		return;
	}

	for (int i = index + 1; i <= N; ++i)
		if (!s[i])
		{
			index = i;
			s[i] = true;
			aux *= p[i];
			Back(k + 1);
			aux /= p[i];
			s[i] = false;
		}
}

void Read()
{
	fin >> N >> K;
	for (int i = 1; i <= N; ++i)
		fin >> p[i];
}