import java.util.Scanner;
public class Main {
	static int[][] gyo2=new int[100000][12];
	static int count=0;
	@SuppressWarnings("resource")
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		long A=sc.nextLong();
		long B=sc.nextLong();
		long C=sc.nextLong();
		long K=sc.nextLong();
		long ans=0;
		if(K>A) {
			ans+=A;
			K-=A;
		}else {
			ans+=K;
			K=0;
		}
		if(K>0) {
			K-=B;
		}
		if(K>0) {
			if(K>C) {
				ans-=C;
			}else {
				ans-=K;
			}
		}
		System.out.println(ans);

}

}

/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