#include <iostream>
#include <fstream>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int N, K, x[30], cnt;
double prob, v[30];

void bt(int k, double p)
{
    if(k <= K)
        for(int i = x[k - 1] + 1; i <= N; ++i)
        {
            x[k] = i;
            bt(k + 1, p * v[i]);
        }
    else
    {
        prob += p;
        cnt++;
    }
}

int main()
{
    f >> N >> K;
    for(int i = 1; i <= N; ++i)
        f >> v[i];
    bt(1, 1);
    g << prob / cnt;
    f.close();
    g.close();
    return 0;
}