import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long a=sc.nextLong(),b=sc.nextLong(),c=sc.nextLong(),k=sc.nextLong();
    long res=0;
    if(k<=a) {
    	System.out.println(k);
    }else if(k<=a+b){
    	System.out.println(a);
    }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