n = gets.to_i
pos = Array.new(n){gets.split(" ").map(&:to_i)}
ans = 0.0
pos.permutation.each do |p|
  (n-1).times do |i|
    ans += ((p[i][0] - p[i+1][0])**2 + (p[i][1] - p[i+1][1])**2) ** 0.5
  end
end
fact = [*1..n].inject{|m, i| m *= i}
puts ans / fact