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

public class Yuki799
{
	public static void Main()
	{
		var abcd = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToList();
		
		int ans = 0;
		
		for(var i = abcd[0]; i <= abcd[1]; ++i)
		{
			for(var j = abcd[2]; j <= abcd[3]; ++j)
			{
				if(i != j) ++ans;
			}
		}
		
		Console.WriteLine(ans);
	}
}