using System;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var lst = Console.ReadLine().Split(' ').
                        Select(s => int.Parse(s)).ToList();
        var red   = Enumerable.Range(lst[0], lst[1] - lst[0] + 1);
        var black = Enumerable.Range(lst[2], lst[3] - lst[2] + 1);

        var total = red.Count() * black.Count() - red.Count(r => black.Contains(r));
        Console.Write("{0}", total);

        return;
    }
}