import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            int A = sc.nextInt();
            int B = sc.nextInt();
            int C = sc.nextInt();
            int K = sc.nextInt();
            if (K <= A) {
                System.out.println(K);
                return;
            }
            if (K <= A + B) {
                System.out.println(A);
                return;
            }
            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