defmodule Main do
  def main do
    1..2
    |> Enum.map(fn _ ->
      IO.read(:line)
      |> String.trim
      |> String.split
      |> Enum.map(&String.to_integer/1)
    end)
    |> solve
    |> IO.puts
  end

  def solve([[xa, ya], [xb, yb]]) do
    (xa * yb + xb * ya) / (xa + xb)
  end
end