n = gets.to_i
x, y = n.times.map{ gets.split.map(&:to_i)}.transpose
pat = [*0..n-1].permutation(n).to_a
sum = pat.map do |l|
  (1..n-1).map do |j|
    Math.sqrt((x[l[j-1]] - x[l[j]])**2 + (y[l[j-1]] - y[l[j]])**2)
  end.inject(:+)
end.inject(:+)
puts sum / pat.size