using System;
using System.Collections.Generic;

namespace Micimacko
{
    internal class Program
    {
        static void Main(string[] args)
        {
            string[] firstLine = Console.ReadLine().Split();
            int Y = int.Parse(firstLine[0]);
            int X = int.Parse(firstLine[1]);

            string[] secondLine = Console.ReadLine().Split();
            int K = int.Parse(secondLine[0]);
            int N = int.Parse(secondLine[1]);

            int remainingHoney = K;

            for (int i = 1; i <= X; i++)
            {
                if (remainingHoney > 0)
                    remainingHoney--;

                if (i == Y)
                    remainingHoney += N;
            }

            Console.WriteLine(remainingHoney);
        }
    }
}