#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int n, p, x[30];
double sum = 0, v[30];

bool valid(int k)
{
    for (int i = 1; i < k; ++i)
    {
        if (x[i] == x[k])
        {
            return 0;
        }
    }
    return 1;
}

void back(int k)
{
    for (int i = 1; i <= n; ++i)
    {
        x[k] = i;
        if (valid(k))
        {
            if (k < p) back(k + 1);
            else
            {
                double pr = 1;
                for (int j = 1; j <= k; ++j)
                {
                    pr *= v[x[j]];
                }
                //fout << pr << ' ';
                sum += pr;
            }
        }
    }
}

int main()
{
    fin >> n >> p;
    for (int i = 1; i <= n; ++i)
    {
        fin >> v[i];
    }
    back(1);
    sum = double(sum / n);
    fout << fixed << setprecision(6) << sum/2 << '\n';
    return 0;
}