import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 文字列の入力
		long a = sc.nextLong();
		long b = sc.nextLong();
		long c = sc.nextLong();
		long k = sc.nextLong();

		long sum = 0;
		if (a >= k) {
			sum = k;
		} else if (a < k && a + b >= k) {
			sum = a;
		} else {
			sum = a - (k - (a + b));
		}

		// 出力
		System.out.println(sum);
		sc.close();
	}
}

/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