N = gets.to_i
xs = Array.new(N)
ys = Array.new(N)
N.times do |i|
  xs[i], ys[i] = gets.split.map(&:to_i)
end

c = Array.new(N){|i| i}

combi = c.permutation.to_a
ans = 0

combi.each do |cm|
  path_length = 0
  for i in 1...cm.length do
    tmp = (xs[cm[i-1]] - xs[cm[i]]) ** 2 + (ys[cm[i-1]] - ys[cm[i]]) ** 2
    tmp = Math.sqrt(tmp)
    path_length += tmp
  end
  ans += path_length
end


puts ans / (combi.length)
