#include <bits/stdc++.h>

using namespace std;

vector<double> proba;
int v[26];
int uz[26];
double res;
int n;
long long C = 0;;
double result = 0;
long long factorial(int nr) {
	long long res = 1;
	for (int i = 1; i <= nr; i++) {
		res = res * i;
	}
	return res;
}

void check(double prod) {
	result += prod;
}

void bt(int t, int k, double prod) {

	if (k == t + 1) {
		check(prod);
		return;
	} else {
		for (int i = v[k - 1] + 1; i <= n; i++) {
			if (uz[i] == 0) {
				prod = prod * proba[i]; 
				uz[i] = 1;
				v[k] = i;
				bt(t, k + 1, prod);
				uz[i] = 0;
				prod = prod / proba[i];
			}
		}
	}
}

double comb(int t) {
	return factorial(n) / (factorial(n - t) * factorial(t));
}

int main()
{	
	ifstream f("dezastru.in");
	ofstream g("dezastru.out");
	int k;
	f >> n >> k;
	proba.push_back(-1);
	for (int i = 0; i < n; i++) {
		double x;
		f >> x;
		proba.push_back(x);
	}
	bt(k, 1, 1);
	result = result * (1 / comb(k));
	g << setprecision(6) << result;
	return 0;		
}