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();
      int score = 0;
      if (K < A) {
        score = K;
      } else if(K <= A + B) {
        score = A;
      } else {
        score = A - (K - A - B); 
      }
      System.out.println(score);
    }
}
/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