n = gets.chomp.to_i

points = []
while line = gets
  x,y = line.strip.split(" ").map(&:to_i)
  points << [x,y]
end

distanse_cathe = {}
distanses = []

points.permutation(n).each do |visittiong_order|
  total_distanse = 0
  visittiong_order.each_with_index do |point,n|
    if n == 0 
      next
    end
    x_div = point[0]-visittiong_order[n-1][0]
    y_div = point[1]-visittiong_order[n-1][1]
    total_distanse += Math.sqrt(x_div * x_div + y_div * y_div)
  end
  distanses << total_distanse
end

puts distanses.inject(&:+) / distanses.length