#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
 ifstream fin("dezastru.in");
 ofstream fout("dezastru.out");
 //fac combinari
int t,n;
long long ns=0;
long double sum=0,a[26];
 int solutie(int k)
 {
     if(k==t)
        return 1;
     return 0;
 }
 void Bk(int b,int k,float p)
 {
     for(int i=b+1;i<=n-t+k;i++)
     {
         if(solutie(k))
                 {
                     sum=sum+p*a[i];
                     ns++;
                 }
         else
            Bk(i,k+1,p*a[i]);
     }
 }
int main()
{
   fin>>n>>t;
    for(int i=1;i<=n;i++)
        fin>>a[i];
    Bk(0,1,1);
   fout<<(float)sum/ns;
    return 0;
}