defmodule Main do
    def count_num(i, a, b, c, d) do
        cond do
            i > b -> 0 
            i >= c && i <= d -> 1 + count_num(i + 1, a, b, c, d)
            True -> count_num(i + 1, a, b, c, d)
        end
    end
    
    def same_number_count(a, b, c, d) do
        count_num(a, a, b, c, d)
    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