class Calc0133
  EPS = 1e-2

  def initialize(args)
    args = args.map { |l| l.chomp.split(/\s+/) }
    @n = args.shift.first.to_i
    @as = args.shift.map(&:to_i)
    @bs = args.shift.map(&:to_i)
  end

  def run
    @as.permutation.map { |as|
      @bs.permutation.count { |bs|
        as.zip(bs).count { |a, b| a > b } > @n / 2
      }
    }.inject(:+).to_f / (1..@n).inject(:*) ** 2
  end
end

puts Calc0133.new(STDIN.readlines).run if __FILE__ == $0