N = gets.to_i
xy = N.times.map { gets.split.map(&:to_i) }

sum = 0
count = 0
(0..(N-1)).to_a.permutation do |p|
  (N - 1).times do |i|
    sum += Math.sqrt((p[i][0] - p[i + 1][0]) ** 2 + (p[i][1] - p[i + 1][1]) ** 2)
  end
  count += 1
end
puts sum / count