#include <iostream>
#include <fstream>
#include <queue>
using namespace std;

#define DIM 30

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");


int N, K;
double p[DIM];
double A[DIM][DIM];
bool s[DIM];


double Sol = 0.0;
double aux = 1.0;
long long int Nr_comb = 1;
int oldIndex;
int index;

void Read();
void Dyn();

int main()
{
	Read();
	
	for (int i = N - K + 1; i <= N; ++i)
		Nr_comb *= i;
	
	for (int i = 1; i <= K; ++i)
		Nr_comb /= i;

	Dyn();

	
	fout << A[N][K] / Nr_comb;

	fin.close();
	fout.close();
	return 0;
}

void Dyn()
{
	A[1][1] = p[1];
	for (int i = 0; i <= N; ++i)
		A[i][0] = 1.0;

	for (int i = 2; i <= N; ++i)
		for (int j = 1; j <= K; ++j)
			A[i][j] = A[i - 1][j] + A[i - 1][j - 1] * p[i];
};

void Read()
{
	fin >> N >> K;
	for (int i = 1; i <= N; ++i)
		fin >> p[i];
}