
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
	// write your code here
        Scanner scanner = new Scanner(System.in);

        int A = scanner.nextInt();
        int B = scanner.nextInt();
        int C = scanner.nextInt();
        int K = scanner.nextInt();

        if(A > K) {
            System.out.println(K);
            return;
        }


        if(A == K) {
            System.out.println(A);
            return;
        }

        if(A + B == K) {
            System.out.println(A);
            return;
        }
        int ans = K - (A+B);
        System.out.println( A - 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