#include <bits/stdc++.h>
using namespace std;

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

int x[26], n, p, nrperm, i;
double v[26], s;

void back(int k, double prod)
{
    for(int i = x[k - 1] + 1; i <= n; ++ i)
    {
        x[k] = i;
        if(k == p)
        {
            s = s + prod * v[x[k]];
            nrperm ++;
        }
        else back(k + 1, prod * v[x[k]]);
    }
}

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

    back(1, 1);
    g << (double)s/nrperm;
    return 0;
}