#include <fstream>
#include <iomanip>

using namespace std;

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

#define n 26

int N, K, st[n], contor;
double p[n], Sol;

bool valid (int level)
{
    for (int i = 1; i < level; i++)
    {
        if (st[level] >= st[i]) return false;
    }
    return true;
}

void use_perm ()
{
    double prob = 1;
    for (int i = 1; i <= K; i++)
    {
        prob *= p[st[i]];
    }
    Sol += prob, ++contor;
}

void back (int level)
{
    for (int i = 1; i <= N; i++)
    {
        st[level] = i;
        if (valid (level) == true)
        {
            if (level == K) use_perm();
            else back (level + 1);
        }
    }
}
int main()
{
    int i;

    in >> N >> K;
    for (i = 1; i <= N; i++) in >> p[i];
    back (1);
    Sol = Sol / contor;
    out << setprecision(6) << Sol;
    return 0;
}