using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var ans = 0;
        for (var i = c[0]; i <= c[1]; ++i) for (var j = c[2]; j <= c[3]; ++j) if (i != j) ++ans;
        WriteLine((c[1] - c[0] + 1) * (c[3] - c[2] + 1) - Math.Max(0, Math.Min(c[1], c[3]) - Math.Max(c[0], c[2]) + 1));
    }
}