#include<stdio.h>
#include<iostream>

using namespace std;

double destroy[32];
int n, k;
double product = 0;
double number = 0;
bool invers = 0;

double total[32][32];

int main() {
	
	freopen("dezastru.in", "r", stdin);
	freopen("dezastru.out", "w", stdout);
	
	cin>>n>>k;
	for( int i = 1; i <= n; ++i) {
		cin>>destroy[i];
		product *= destroy[i];
	}
	
	total[0][0] = 1;
	for( int j = 0; j < k; ++j) {
		for( int i = 0; i < n ; ++i)
			if( total[j][i] != 0 ){
				for( int l = i + 1; l <= n; ++l)
					total[j+1][l] += total[j][i] * destroy[l];
			}
		}
	
	for( int j = 0; j <= n; ++j)
		product += total[k][j];
	
	long long number = 1;
	if( k < n/2)
		k = n - k;
	for( int i = k + 1; i <= n; ++i)
		number *= i;
	for( int i = 1; i <= n - k; ++i)
		number /= i;
	cout<< product/number;
	
	return 0;
}