#include <bits/stdc++.h>

using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int combinari[30];
int n, p;
double cost[30];
double nr, prod;

void back(int k,double  pro)
{
  for(int i = combinari[k - 1] + 1;i <= n;++i)
  {
    combinari[k] = i;
    if(k == p)
    {
      prod += pro * cost[combinari[k]];
      ++nr;
    }
    else
      back(k + 1, pro * cost[combinari[k]]);
  }
}

int main()
{
  f>>n>>p;
  for(int i = 1;i <= n;++i)
    f>>cost[i];
  back(1, 1);
  g<<(double)prod / nr;
}