using System;
using System.Linq;

class Program
{
    public void Solve()
    {
        int[] ABCD = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        int count = 0;
        foreach (var red in Enumerable.Range(ABCD[0], ABCD[1] - (ABCD[0] - 1)))
        {
            foreach (var black in Enumerable.Range(ABCD[2], ABCD[3] - (ABCD[2] - 1)))
            {
                if (red == black) { continue; }
                count++;
            }
        }
        Console.WriteLine(count);
    }

    static void Main()
    {
        var solver = new Program();
        solver.Solve();
    }
}