#!/usr/bin/ruby

n = gets.to_i
$x = Array.new(n)
$y = Array.new(n)

(0..n-1).each do |i| 
  $x[i],$y[i] = gets.split.map(&:to_i)
end

def dist(i,j)
  tmp = ($x[i]-$x[j])**2 + ($y[i]-$y[j])**2
  return Math.sqrt(tmp)
end

def fact(n)
  return 1 if n == 1
  return n * fact(n-1)
end

# main
sum = 0 

(0..n-1).each do |i| 
  (i..n-1).each do |j| 
    sum += dist(i,j)*fact(n-1)*2
  end 
end

puts sum/fact(n)
