N = gets.to_i
x = Array.new(N)
y = Array.new(N)
N.times do |i|
  x[i],y[i] = gets.split.map(&:to_i)
end
dist = 0
dist_num = 0
[*0..N-1].permutation(N).to_a.each_with_index do |z, index|
  dist_num += 1
  (z.length-1).times do |i|
    node1 = z[i]
    node2 = z[i+1]
    dist += Math.sqrt( (x[node1]-x[node2])**2 + (y[node1]-y[node2])**2 )
  end
end
puts dist / dist_num