#include <iostream>
#include <stdio.h>
using namespace std;
int n,k;
double prob[30];
double matrice[30][30];
int pascal[30];

void triunghiul_pascal()
{
    for(int i = 0 ; i < n ; i++)
        pascal[i] = 1;
    for(int i = 2 ; i <= n ; i++)
        for(int j = i - 1 ; j > 0 ; j --)
         pascal[j] = pascal[j] + pascal[j-1];
}

void rezolv()
{
    for(int i = 1 ; i <= n ; i++)
    {
        for(int j = 1 ; j <= k ; j++)
            matrice[i][j] = matrice[i-1][j] +  matrice[i-1][j-1]*prob[i-1];
    }
}

void read()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d %d",&n,&k);
    for(int i = 0 ; i <= n ; i++)
    {
        scanf("%lf ",&prob[i]);
        matrice[i][0] = 1;
    }

}

int main()
{
    read();
    triunghiul_pascal();
    rezolv();
    printf("%lf",matrice[n][k]/pascal[k]);
    return 0;
}