import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        final Scanner scanner = new Scanner(System.in);
        int a, b, c, k;

        a = scanner.nextInt();
        b = scanner.nextInt();
        c = scanner.nextInt();
        k = scanner.nextInt();

        if(k <= a) {
            System.out.println(k);
        }
        else if(k <= a+b) {
            System.out.println(k-b);
        }
        else if (k <= a+b+c) {
            System.out.println(a-(k-(a+b)));
        }
    }
}
/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