using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Micimacko
{
    internal class Program
    {
        static void Main(string[] args)
        {
            int x = 0;
            int y = 0;
            int k=0;
            int n=0;
            int m = 0;

            string[] be = Console.ReadLine().Split();
            y=int.Parse(be[0]);
            x=int.Parse(be[1]);
            be = Console.ReadLine().Split();
            k=int.Parse(be[0]);
            n=int.Parse(be[1]);

            if (k > y)
                m = k - y;
            else
                m = 0;

            if (m+n > x-y)
                m=m+n-(x-y);
            else
                m=0;
            Console.WriteLine(m);
        }
    }
}