import java.util.Scanner;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
 
		int A,B,C,K;
		A=sc.nextInt();
		B=sc.nextInt();
		C=sc.nextInt();
		K=sc.nextInt();
		int sum=A;
		if(A>=K) {
			sum = K;
		}
		else if(A+B>=K) {
			sum=A;
		}else {
			sum=A-(K-A-B);
		}
		System.out.println(sum);
		
	}
}
 
 
 
/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