/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		
	Scanner  sc = new Scanner(System.in);
	int n = sc.nextInt();  
            
            if(n < 1000){ 
                    System.out.println("ABC");	
            }
            
            else{
            	System.out.println("ABD");
            }
	
		// your code goes here
	}
}