import java.util.Scanner;

public class Main{
    public static void main(String args[]){

    Scanner scan = new Scanner(System.in);

    int a = scan.nextInt();
    int b = scan.nextInt();
    int c = scan.nextInt();
    int k = scan.nextInt();
    
    
        if (k <= a) {
            System.out.println(k);
        } else if (k > a && k <= a + b) {
            System.out.println(a);
        }  else if (k > a + b && k <= a + b + c) {
            System.out.println(a- (k-a-b));
        } 
    
      


      scan.close();
    }
  }
/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