import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		int C = stdIn.nextInt();
		int K = stdIn.nextInt();
		int ans = 0;
		for(int a = 0; a < K; a++){
			if(A > 0){
				A = A - 1;
				ans++;
			}
			else if(B > 0){
				B = B - 1;
			}
			else{
				C = C - 1;
				ans = ans - 1;
			}
		}
		System.out.println(ans);
		stdIn.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