#include <fstream>

using namespace std;

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

int N, K, x[26], cnt;
double prob, v[26];

void backt(int k, double p)
{
	if(k > K)
	{
		cnt++;
		prob += p;
	}
	else
		for(int i = x[k - 1] + 1; i <= N; i++)
		{
			x[k] = i;
			backt(k + 1, p * v[i]);
		}
}

int main()
{
	f >> N >> K;
	for(int i = 1; i <= N; i++)
		f >> v[i];
	backt(1, 1);
	g << prob / cnt;
	f.close();
	g.close();
	return 0;
}