import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		long A = sc.nextInt();
		long B = sc.nextInt();
		long C = sc.nextInt();
		long K = sc.nextInt();
		long ans = 0;

		if (A < K){
			ans = A;
			K -= A;
		} else {
			ans = K;
			System.out.println(ans);
			return;
		}

		if (B < K){
			K -= B;
		} else {
			System.out.println(ans);
			return;
		}

		ans += -1 * K;
		System.out.println(ans);
	}

}

/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