#include <fstream>

using namespace std;

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

int n, k;
int x[26];
long double p[26], fact[26];
long double rez, nr;

void bkt(int niv)
{
    for (int i=x[niv-1]+1; i<=n; i++)
    {
        x[niv] = i;
        if (niv == k)
        {
            long double prod = 1;
            for (int j=1; j<=k; j++)
                prod *= p[x[j]];
            rez += prod * fact[k];
            nr += fact[k];
        }
        else
            bkt(niv+1);
    }
}

int main()
{
    f >> n >> k;
    for (int i=1; i<=n; i++)
        f >> p[i];

    fact[1] = 1;
    for (int i=2; i<=k; i++)
        fact[i] = fact[i-1] * i;

    bkt(1);

    rez /= nr;

    g << rez;

    return 0;
}