class Calc0066
  EPS = 1e-6

  def initialize(args)
    args = args.map { |l| l.chomp.split(/\s+/) }
    @m = args.shift.first.to_i
    @ss = args.map(&:first).map(&:to_i)
    @memo = Array.new(2 ** @m) { [1] + Array.new(@m) }
  end

  def calc(i, k)
    @memo[i][k] = (members(i, k) - members(i, k - 1)).map { |j|
      @memo[j][k - 1] * win_p(i, j)
    }.inject(:+) * @memo[i][k - 1]
  end

  def members(i, k)
    s = (i / 2 ** k)
    ((s * 2 ** k)...((s + 1) * 2 ** k)).to_a
  end

  def win_p(i, j)
    (@ss[i] ** 2).to_f / (@ss[i] ** 2 + @ss[j] ** 2)
  end

  def run
    (1..@m).each do |k|
      (0...(2 ** @m)).each do |i|
        calc(i, k)
      end
    end
    @memo[0][@m]
  end
end

puts Calc0066.new(STDIN.readlines).run if __FILE__ == $0