using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var a = int.Parse(line[0]);
        var b = int.Parse(line[1]);
        var c = int.Parse(line[2]);
        var d = int.Parse(line[3]);
        var count = 0;
        for (int i = a; i <= b; i++)
            for (int j = c; j <= d; j++)
                if (i != j) count++;
        Console.WriteLine(count);
    }
}