N = STDIN.gets.to_i

towns = []

N.times do
  x, y = gets.split.map(&:to_i)
  towns << [x, y]
end

length = 0.0
size = 0
(0..(N-1)).to_a.permutation(N) do |p|
  size += 1
  (0..(N-2)).to_a.each do |i|
    x1 = towns[p[i]][0]
    y1 = towns[p[i]][1]
    x2 = towns[p[i + 1]][0]
    y2 = towns[p[i + 1]][1]

    # p "x1: #{x1} y1: #{y1} x2: #{x2} y2: #{y2} "
    # p "#{p[i]} -> #{p[i + 1]}"

    length += Math.sqrt((x1 - x2)**2 + (y1 - y2)**2)
  end
end

puts length / size
