#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;

#define maxN 30

double DP[maxN][maxN], P[maxN];


int main()
{
	freopen ("dezastru.in", "r", stdin);
	freopen ("dezastru.out", "w", stdout);
	
	int N, K;
	scanf ("%d %d", &N, &K);
	
	for (int i = 1; i <= N; ++ i) scanf ("%lf", &P[i]);
	
	for (int i = 0; i <= N; ++ i) DP[i][0] = 1;
	
	for (int i = 1; i <= N; ++ i)
		for (int j = 1; j <= K && j <= i; ++ j) DP[i][j] = DP[i - 1][j] + DP[i - 1][j - 1] * P[i];
	
	for (int i = 1; i <= N - K; ++ i) DP[N][K] *= i;
	for (int i = K + 1; i <= N; ++ i) DP[N][K] /= i;
	
	printf ("%.7lf", DP[N][K]);
	
	return 0;
}