import java.util.*;
public class Main {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int a = in.nextInt();
		int b = in.nextInt();
		int c = in.nextInt();
		int k = in.nextInt();
		int total =0;
		total += Math.min(a, k);
		k -= Math.min(a, k);
		if (k==0) {
			System.out.println(total);}
		else {
			k-= Math.min(b, k);
			if (k==0) {
				System.out.println(total);}
			else {
				total -= Math.min(c, k);
				k -= Math.min(c, k);
				System.out.println(total);}}
	}
	
}
/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