#include <fstream>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double v[26], fact[26], d[26][26];
int main()
{
	int n, k;
	f >> n >> k;
	for (int i = 1; i <= n; i++)
		f >> v[i];
	fact[0] = 1;
	for (int i = 1; i <= n; i++)
		fact[i] = fact[i - 1] * i;
	d[0][0] = 1;
	for (int i = 1; i <= n; i++)
	{
		d[i][0] = 1;
		for (int j = 1; j <= i; j++)
			d[i][j] = d[i - 1][j] + d[i - 1][j - 1] * v[i];
	}
	g << d[n][k] * fact[k] * fact[n - k] / fact[n];
	return 0;
}