n = gets.to_i
s = readlines.map { |e| e.split(' ').map(&:to_f) }
result = 0.0
all_order = s.permutation(n).to_a
all_order.each do |el|
  el.each_with_index do |line, i|
    next if i == el.size - 1

    next_line = el[i + 1]
    x, y = line
    nx, ny = next_line
    result += Math.sqrt(((x - nx) * (x - nx)) + ((y - ny) * (y - ny)))
  end
end

p result / all_order.size.to_f
