using System;
using System.Linq;
using System.Collections.Generic;

static class NagataKanaIsAGenius {
    public static void Main() {
        int[]abcd=Console.ReadLine().Split().Select(int.Parse).ToArray();
        int a=abcd[0];
        int b=abcd[1];
        int c=abcd[2];
        int d=abcd[3];
        Console.WriteLine((b-a+1)*(d-c+1)-Math.Max(0,Math.Min(b,d)-Math.Max(a,c)+1));
    }
}