def calculate_distance(before, after)
  return Math.sqrt((before[0] - after[0]) ** 2 + (before[1] - after[1]) ** 2)
end

TOWNS = gets.to_i
POSITIONS = []
TOWNS.times{ POSITIONS << gets.split(' ').map(&:to_i) }
total_d = 0

orders = []
TOWNS.times{ |i| orders << i }
orders.permutation(TOWNS) do |order|
  (TOWNS - 1).times do |i|
    before = POSITIONS[order[i]]
    after = POSITIONS[order[i + 1]]
    total_d += calculate_distance(before, after)
  end
end
print total_d / (1..TOWNS).inject(1,:*)