#include <fstream>
#include <iomanip>

using namespace std;

double sum = 0;
int num = 0;
int n, k;
double v[26];

inline void back(int count, int i, double product)
{
	if (count == k)
	{
		sum += product;
		num++;
		return;
	}
	for (int j = i; j < n - k + count + 1; j++)
	{
		back(count + 1, j + 1, product * v[j]);
	}
}

int main()
{
	ios_base::sync_with_stdio(false);
	ifstream in("dezastru.in");
	ofstream out("dezastru.out");

	in >> n >> k;
	for (int i = 0; i < n; i++)
	{
		in >> v[i];
	}

	back(0, 0, 1.0);

	out << setprecision(7) << sum / num;

	return 0;
}