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

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

void back(int q)
{
    for (int i = st[q - 1] + 1; i <= n; i++)
    {
        st[q] = i;
        prod *= v[i];
        if (q < k)
            back(q + 1);
        else
        {
            rez += prod;
            nr++;
        }
        prod /= v[i];
    }
}

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