    import java.util.Scanner;
     
     
    public class Main {
        

        public static void main(String[] args) {
            Scanner sc = new Scanner(System.in);
            int a = sc.nextInt();
            int b = sc.nextInt();
            int c = sc.nextInt();
            int k = sc.nextInt();
 
                    if (a >= k) {
            System.out.println(k);
        } else if (a + b >= k) {
            System.out.println(a);
        } else if (a + b + c >= k) {
            System.out.println(a - (c - ((a + b + c) - k)));
        }

            
            
        }
    }
/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