#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int p[30], u[30], n, m;
int l = 1;
float v[30],cal[30];
void Afisare()
{
	float sum = 1;
	for (int i = 1; i <= m; i++)
		sum = sum * float(v[p[i]]);
	cal[l++] = sum;
}

void Generare(int k)
{
	if (k - 1 == m)
		Afisare();
	else
	{
		for (int i = 1; i <= n; i++)
		{
			if (!u[i])
			{
				u[i] = 1;
				p[k] = i;
				Generare(k + 1);
				u[i] = 0;
			}
		}
	}
}

int main()
{
	fin >> n >> m;
	for (int i = 1; i <= n; i++)
		fin >> v[i];
	Generare(1);
	l--;
	float sumx = 0;
	for (int i = 1; i <= l; i++)
		sumx = sumx + cal[i] / float(l);
	fout <<setprecision(6)<<sumx;
	return 0;
}