class Program
{
    static void Main(string[] args)
    {
        string[] input = Console.ReadLine()!.Split(' ');
        int redMin = int.Parse(input[0]);
        int redMax = int.Parse(input[1]);
        int blackMin = int.Parse(input[2]);
        int blackMax = int.Parse(input[3]);
        int count = 0;

        for (int i = redMin; i <= redMax; i++)
        {
            for (int j = blackMin; j <= blackMax; j++)
            {
                if (i != j)
                {
                    count++;
                }
            }
        }

        Console.WriteLine(count);
    }
}