#include <fstream>
#include <algorithm>
#define x first
#define y second

using namespace std;

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

int n, N, i, k;
pair <double,int> v[30];
double sum,prod,nr;


int main()
{
    fin >> n >> k;

    for(i = 1; i <= n; ++i)
        {
            fin >> v[i].x;
            v[i].y = i;
        }

    prod = 1;

    for(i = 1; i <= k; ++i)
        prod*=v[i].x;

    sum+=prod;
    ++nr;

    while(next_permutation(v+1,v+n+1))
        {
            prod = 1;
            for(i = 1; i <= k; ++i)
                prod*=v[i].x;
            sum+=prod;
            ++nr;
        }
    fout.precision(6);
    fout  << sum/nr;

    return 0;
}