import java.util.*;

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 sum = Math.min(a,k) + 0*(Math.min(k-Math.min(a,k),b)) + (-1)*(k-Math.min(a,k)-Math.min(k-Math.min(a,k),b));
  System.out.println(sum);
  
  sc.close();
}//void
}//class
/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