def fact(n)
  return 1 if n == 0
  return n * fact(n-1)
end

def distance(p1, p2)
  x1 = p1[0]
  y1 = p1[1]
  x2 = p2[0]
  y2 = p2[1]
  return Math.sqrt( (x1-x2)**2 + (y1-y2)**2 )
end

def main
  n = gets.to_i
  ps = []
  n.times do
    point = gets.chomp.split(" ").map(&:to_i)
    ps.push point
  end
  dist = 0
  (0..n-1).to_a.permutation do |perm|
    (0..n-2).each do |i|
      dist += distance(ps[perm[i]], ps[perm[i+1]])
    end
  end
  puts dist * 1.0 / fact(n)
end

main

