N = gets.to_i
X,Y = Array.new(N){gets.split.map &:to_i}.transpose
C = (1..N).inject(:*)

sum = 0.0
(0..N-1).to_a.permutation.each do |ix|
  len = 0.0
  ix.each_cons(2) do |i,j|
    x = Math.sqrt((X[i]-X[j])**2 + (Y[i]-Y[j])**2)
    len += x
  end
  sum += len
end

puts sum / C
