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

  defp solve([[px, py], [qx, qy]]) do
    (((px - qx) |> abs) + ((py - qy) |> abs)) / 2
  end
end