defmodule Main do
    def same_number_count(a, b, c, d) do
        n1 = max(a, c)
        n2 = min(b, d)
        max(0, n2 - n1 + 1)
    end

    def solve(a, b, c, d) do
        (b - a + 1) * (d - c + 1) - same_number_count(a, b, c, d)
    end

    def main do
        [a, b, c, d] = IO.gets("") |> String.trim |> String.split |> Enum.map(&String.to_integer/1)
        solve(a, b, c, d) |> IO.puts
    end
end