n = gets.to_i
point = []
n.times { point << gets.split.map(&:to_i) }
sum = 0.0
cnt = 0
point.permutation do |x|
  dist = 0.0
  i = 1
  while i < x.size
    dist += ((x[i - 1][0] - x[i][0])**2 + (x[i - 1][1] - x[i][1])**2)**0.5
    i += 1
  end
  sum += dist
  cnt += 1
end
puts sum/cnt
