
import java.util.*;

public class Main {// Main
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
      long a =sc.nextLong();
      long b =sc.nextLong();
      long c =sc.nextLong();
      long k =sc.nextLong();
     
      
       if(k <= a+b ) {
    	   System.out.println(Math.min(a, k));
       }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