/*
    If you can't explain it simply, you don't understand it well enough.
*/

#include <cstdio>

using namespace std;

const int Nmax = 30;

int n, k , i , x , nr;

double sum;
double p[Nmax];

void generare(int niv , int ind , double probability)
{
    if (niv == k + 1)
    {
        sum += probability;
        nr++;
        return;
    }
    
    for (int i = ind + 1; i <= n - k + niv; ++i)
        generare(niv + 1 , i , probability * p[i]);
}

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    
    scanf("%d %d", &n, &k);
    
    for (i = 1; i <= n; ++i)
        scanf("%lf", &p[i]);
    
    generare(1 , 0 , 1);
    
    printf("%.6lf", sum / nr);
    
    return 0;
}