n = gets.chomp.to_i
xy = []
n.times do |i|
  xy[i] = gets.chomp.split.map(&:to_f)
end

sum = 0.0
(0..(n-2)).each do |i|
  ((i+1)..(n-1)).each do |j|
    sum += ((xy[i][0] - xy[j][0])**2 + (xy[i][1] - xy[j][1])**2)**0.5
  end
end

ans = sum * 2 / n

puts(ans)