#include <iostream>
#include "stdio.h"
using namespace std;
double choose[26] = {0 , 1}, dp[26][26], p[26];


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 = 2; i <= n;i++)
		choose[i] = i * choose[i - 1];
	for(int i = 0; i <= n;i++)
		dp[i][0] = 1;
	for(int i = 1; i <= n;i++)
		for(int j = 1; j <= n;j++)
			dp[i][j] = dp[i-1][j-1] * p[i] + dp[i-1][j];
	
	printf("%.6f", dp[n][k]/(choose[n]/(choose[n-k]*choose[k])));
	return 0;
}