N = gets.to_i
XY = N.times.map{gets.split.map(&:to_i)}
sum = 0
cnt = 0

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