using System;
using System.Collections.Generic;

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);
    }
}