n = gets.to_i
hash_xy = {}
n.times do |i|
  hash_xy[i + 1] = gets.split.map(&:to_i)
end

arrays = (1..n).to_a
distance = 0
arrays.permutation do |order|
key_first = order.first
  order.each do |key|
    if key == key_first
      $key_before = key.to_i
    else
      x = hash_xy[key][0] - hash_xy[$key_before][0]
      y = hash_xy[key][1] - hash_xy[$key_before][1]
      $key_before = key
      distance += Math.sqrt(x**2 + y**2)
    end
  end
end

def factorial(number)
  if number.zero?
    1
  else
    number * factorial(number - 1)
  end
end

p distance / factorial(n)