n = gets.to_i

x = Array.new(n)
y = Array.new(n)
n.times do |i|
  x[i], y[i] = gets.chomp.split.map(&:to_i)
end
num = 0
sum = 0
(1..n).to_a.permutation.to_a.each do |a|
  (a.length - 1).times do |i|
    sum += Math.sqrt((x[a[i]-1] - x[a[i-1]-1]) ** 2 + (y[a[i]-1] - y[a[i-1]-1]) ** 2)
  end
  num += 1
end

p sum / num