#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];


long long int Nr_comb = 1;


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();

	FILE *f = fopen("dezastru.out", "w");
	fprintf(f, "%.7lf", A[N][K] / (double)Nr_comb);

	fin.close();
	fclose(f);
	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];
};

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