class Yukicoder
  def initialize
    x1, y1 = gets.chomp.split.map(&:to_i)
    x2, y2 = gets.chomp.split.map(&:to_i)

    if y1 > y2
      a = x1 + x2
      b = y1 - y2

      puts (y1 - Rational(b, a) * x1).to_f
    else
      a = x1 + x2
      b = y2 - y1

      puts (y2 - Rational(b, a) * x2).to_f
    end
  end
end

Yukicoder.new