n = gets.to_i
arr = []
n.times {arr << gets.strip.split.map(&:to_i)}
sum = 0
ps = []

(0..n-1).to_a.permutation(n) do |arr|
  ps << arr
end

ps.each do |nums|
  (nums.length-1).times do |num|
    cx = arr[nums[num]][0]
    cy = arr[nums[num]][1]
    nx = arr[nums[num+1]][0]
    ny = arr[nums[num+1]][1]
    dx = (nx - cx) ** 2
    dy = (ny - cy) ** 2
    d = dx + dy
    sum += Math.sqrt(d)
  end
end

p sum.to_f / ps.size
