import java.util.Scanner;

public class Main {

  /**
   * @param args
   */
  public static void main(String[] args) {
    // TODO 自動生成されたメソッド・スタブ
    Scanner cin = new Scanner(System.in);
    int A = cin.nextInt();
    int B = cin.nextInt();
    int C = cin.nextInt();
    int K = cin.nextInt();

    int ans = 0;
    for (int i=0;i<A;i++){
      K = K - 1;
      if (K < 0) break;
      ans = ans + 1;
    }
    for (int i=0;i<B;i++){
      K = K - 1;
      if (K < 0) break;
      ans = ans + 0;
    }
    for (int i=0;i<C;i++){
      K = K - 1;
      if (K < 0) break;
      ans = ans - 1;
    }
    System.out.println(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