#include <fstream>
#include <algorithm>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");
short int n,k,nr;
float a[25],p=1,s;
char x[25];

void program();
void citire();
void backtrack();

int main()
{
    citire();
    backtrack();
    g<<s/nr;

return 0;
}


void citire()
{
     f>>n>>k;

 for(int i=0;i<n;i++)
     f>>a[i];
}

void backtrack()
{
 for(int i=0;i<n;i++)
    {x[i]=i;

    }
 do
 {
  program();
 }while(next_permutation(x,x+n));

}

void program()
{
 for(int i=0;i<k;i++)
   {
     p*=a[x[i]];
   }
 s+=p;
 nr++;
 p=1;

}