defmodule Main do
  def input, do: IO.read(:line) |> String.trim
  def ii, do: input() |> String.to_integer
  def li, do: input() |> String.split |> Enum.map(&String.to_integer/1)
  def yn(b), do: IO.puts(if b, do: "Yes", else: "No")

  def main do
    [x1, y1] = li()
    [x2, y2] = li()

    [x3, y3] = [-x1, y1]
    a = x1
    b = x2

    # x = (x3 * b + x2 * a) / (x3 + x2)
    y = (y3 * b + y2 * a) / (a + b)
    IO.puts y
  end
end