import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int A = Integer.parseInt(scan.next());
        int B = Integer.parseInt(scan.next());
        int C = Integer.parseInt(scan.next());
        int K = Integer.parseInt(scan.next());

        if (A >= K) {
            System.out.println(K);
        } else if ((A + B) >= K) {
            System.out.println(A);
        } else {
            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