n = gets.chomp.to_i
points = []
route = 1
n.times do |num|
  x, y = gets.chomp.split.map(&:to_f)
  points << [x + 1000, y + 10000]
  route *= num + 1
end

streets = route * (n - 1)
street_pattern = n * (n -1) / 2

all_length = 0
for i in 0..(n - 1) do
  for j in i..(n - 1) do
    all_length += Math.sqrt((points[i][0] - points[j][0]) ** 2 + (points[i][1] - points[j][1]) ** 2) * (streets / street_pattern)
  end
end
puts all_length / route