#include <fstream.h>
#include <stdio.h>

ifstream fin ("dezastru.in");

double a[48];
int  n,K;
int nr=0;
double P=0,S=0,R;

void citire()
{
  fin>>n>>K;
    for (int i=0;i<n;i++)
      fin>>a[i];
  fin.close();
}

void back (int k,int poz,double S)
{
  if (k==K)
  {
     P+=S;
     nr++;
     return ;
  }
  for (int i=poz+1;i<n-K+k+1;i++)
      back (k+1,i,S*a[i]);
}

int main ()
{
  citire();
  back(0,-1,1);
  P/=nr;
  freopen ("dezastru.out","w",stdout);
  printf("%.6lf",P);
  fclose (stdout);
  return 0;
}