defmodule Main do
  def main do
    IO.gets("")
    |> String.split()
    |> Enum.map(&String.to_integer(&1))
    |> (fn [a, b, c, d] ->
          (b - a + 1) * (d - c + 1) -
            cond do
              b < c -> 0
              true -> min(b, d) - max(a, c) + 1
            end
        end).()
    |> IO.puts()
  end
end