#include <iostream>
#include <iomanip>
#include <stdio.h>
#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];
double C[DIM];


void Read();
void Dyn();
double Comb();


int main()
{
	Read();
	
	Dyn();

	fout << A[N][K] / Comb();
	fin.close();
	fout.close();
	return 0;
}

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

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

double Comb()
{
	C[0] = 1;
	for (int i = 1; i <= N; ++i)
		for (int j = i; j >= 1; --j)
			C[j] += C[j - 1];
	return C[K];
}

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