n = gets.to_i
k = [1,1,2,6,24]
a = gets.split.map(&:to_i)
b = gets.split.map(&:to_i)
ans = 0
a.permutation(n).to_a.each{|x|
    b.permutation(n).to_a.each{|y|
        a_win = 0
        n.times{|i|
            if x[i] > y[i]
                a_win += 1
            end
            }
        if a_win > n / 2
            ans += 1
        end
        }
    }
p ans * 1.0 / (k[n] * k[n])