import java.util.*;

class Main {
  static Scanner sc = new Scanner(System.in);

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