#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];
};