import java.util.Scanner;
public class Main {
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        int c = scanner.nextInt();
        int k = scanner.nextInt();

        int sum = 0;

        if(a <= k){
            sum += a;
            k = k - a;
            if(k == 0){
                System.out.println(sum);
                return;
            }
        }else{
            sum += k;
            System.out.println(sum);
            return;
        }

        if(b <= k){
            k = k - b;
            if(k == 0){
                System.out.println(sum);
                return;
            }
        }else{
            System.out.println(sum);
            return;
        }

        sum -= k;
        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