import java.util.Scanner;
class Main{
	public static void main(String[] args){
    Scanner sr = new Scanner(System.in);
      int A = sr.nextInt();
      int B = sr.nextInt();
      int C = sr.nextInt();
      int K = sr.nextInt();
      //System.out.print(A +""+ B +""+C+ ""+K );
      if ( K <= A)
        System.out.print(K);
      else
        if(K <= A+B)
          System.out.print(A);
        else
          System.out.print(2*A+B-K); 
    }
}
/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