class Start
  def main
    a, b = 2.times.map { input.split.map { |it| it.to_i } }
    solve(a, b)
    nil
  end

  private

  def solve(a, b)
    res = 0
    res += a[1] * b[0]
    res += a[0] * b[1]
    res /= (a[0] + b[0]).to_f
    echo res.round(6)
  end

  protected

  def echo(*x)
    print(*x)
    puts
  end

  def input = gets.chomp
end

Start.new.main