N = gets.to_i
A = gets.split.map(&:to_i)
B = gets.split.map(&:to_i)

n = 0
d = 0
B.permutation do |b|
  w = A.zip(b).select{|a| a[0] > a[1]}.size
  l = N - w
  n += 1 if w > l
  d += 1
end

puts Rational(n, d).to_f