n = gets.to_i
cities = Array.new
n.times do
  cities << gets.split.map(&:to_i)
end
sum = Array.new
cities.permutation(n).each do |p|
  current_pos = p.shift
  tmp = 0
  p.each do |next_pos|
    tmp += Math.sqrt((current_pos[0] - next_pos[0]) ** 2 + (current_pos[1] - next_pos[1]) ** 2)
    current_pos = next_pos
  end
  sum << tmp
end
puts sum.inject(:+) / sum.length
