n = gets.to_i
a = []
n.times do |_i|
  x, y = gets.split.map(&:to_i)
  a << [x, y]
end

total = 0
count = 0
a.permutation do |list|
  count += 1
  _x = _y = -1
  list.each do |x, y|
    if _x != -1
      add  = Math.sqrt(((_x - x)**2) + ((_y - y)**2))
      total += add
    end
    _x = x
    _y = y
  end
end

puts format('%.14f', 1.0 * total / count)
