#include <iostream>
#include <fstream>
using namespace std;
ifstream f ("dezastru.in");
ofstream g ("dezastru.out");

double v[27], prod = 1, rez = 0, a[27][27];
int n, k, nr, fact[27];

int main()
{
    f >> n >> k;
    fact[0] = 1;
    a[0][0] = 1;
    for (int i = 1; i <= n; i++)
    {
        f >> v[i];
        fact[i] = fact[i - 1] * i;
        a[i][0] = 1;
    }
    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] * v[i];
    nr = fact[n] / fact[k] / fact[n - k];
    rez = a[n][k];
    g << rez / (double)nr;
    return 0;
}