#include <iostream>
#include <fstream>

using namespace std;

ifstream in("dezastru.in");
ofstream out("dezastru.out");

int n, m;
double val[26];
int st[26], nrDez;
bool viz[26];
double dez[26];

void citire(){
  in >> n >> m;
  for(int i = 1; i <= n; i++)
    in >> val[i];
}

void bkt(int k){
  for(int i = 1; i <= n; i++)
    if(viz[i] == false){
      st[k] = i;
      viz[i] = true;
      if(k == n){
        nrDez++;
        dez[nrDez] = val[st[1]] * val[st[2]];
      }
      else
        bkt(k + 1);
      viz[i] = false;
    }
}

void rezolvare(){
  double raspuns = 0;
  for(int i = 1; i <= nrDez; i++)
    raspuns += (dez[i] / nrDez);
  out << raspuns;
}

int main(){
  citire();
  bkt(1);
  rezolvare();
  return 0;
}