N = gets.chomp.to_i
x,y = [],[]
1.upto(N) do |i|
  x[i],y[i] = gets.chomp.split.map(&:to_i)
end

sum = 0
(1..N).to_a.permutation(N) do |arr|
  (N-1).times do |n|
    start = arr[n]
    goal = arr[n+1]
    sum += (Math.sqrt((x[start]-x[goal])**2 + (y[start]-y[goal])**2))
  end
end

k = (1..N).to_a.inject(:*)

p sum.to_f / k
