n = gets.to_i
lists = []
(0..n-1).each do |i|
  inputs = gets.split.map(&:to_i)
  lists << inputs
end
length = 0
(0..n-2).each do |i|
  k = i + 1
  while k < n do
    length += ((lists[i][0] - lists[k][0])**2 + (lists[i][1] - lists[k][1])**2) ** 0.5
    k += 1
  end
end
puts length * 2 / n
