//copypaster
#include<bits/stdc++.h>
using namespace std;
int N, K;
double ans, dv,a[200],A[30][30];
int main()
{
	ifstream cin("dezastru.in");
	ofstream cout("dezastru.out");
	cin >> N >> K;
	for (int i = 1; i <= N; i++)
	{
		cin >> a[i];
		A[i][0] = 1;
	}
	A[0][0] = 1;
	for (int i = 1; i <= N; i++)
		for (int j = 1; j <= i; j++)
			A[i][j] = A[i-1][j-1]*a[i] + A[i-1][j];
	double d = A[N][K],f = 1;
	for (int i = K+1; i <= N; i++) f*=i;
	for (int i = 2; i<= N-K; i++) f/=i;
	cout << setprecision(6) << d/f;
}