n = gets.to_i
town = n.times.map{gets.split.map(&:to_i)}

sum_val = 0
cnt = 0
(0..(n-1)).to_a.permutation do |numbers|
  cnt += 1
  (numbers.size-1).times do |i|
    sum_val += ((town[numbers[i]][0] - town[numbers[i+1]][0])**2 + (town[numbers[i]][1] - town[numbers[i+1]][1])**2)**(1/2.0)
  end
end

puts sum_val/cnt