import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		Integer a = sc.nextInt();
		Integer b = sc.nextInt();
		Integer c = sc.nextInt();
		Integer k = sc.nextInt();
		
		Integer result = 0;
		if(k <= a) {
			result = k;
		} 
		else if(k <= a+b) {
			result = a;
		}
		else{
			result = a - (k - a - b);
		}
		System.out.println(result);
	}
}
/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