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