import java.util.*;

public class Main {

    public static void main(String args[]) {


        Scanner scan = new Scanner(System.in);

        int a = scan.nextInt();
        int b = scan.nextInt();
        int c = scan.nextInt();
        int k = scan.nextInt();

        int d = a + b;

        if (a >= k) {
            System.out.println(String.valueOf(k));
            return;
        }


            if (d >= k) {
                System.out.println(String.valueOf(a));
            } else {
                int e = k - d;
                System.out.println(String.valueOf(a - e));
            }
        }
    

}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz