import java.util.*;
public class Main {
	public static void main(String arg[]) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		int C = sc.nextInt();
		int K = sc.nextInt();

		int res = 0;

		res += (K >= A) ? A : K;
		K -= A;
		if(K > 0) {
			K-= B;
		}
		if(K > 0) {
			res -= (K >= C) ? C : K;
		}
		System.out.println(res);
	}
}
/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