Problem:
Score : 100 points

Problem Statement

Decades have passed since the beginning of AtCoder Beginner Contest.

The contests are labeled as ABC001, ABC002, ... from the first round, but after the 999-th round ABC999, a problem occurred: how the future rounds should be labeled?

In the end, the labels for the rounds from the 1000-th to the 1998-th are decided: ABD001, ABD002, ..., ABD999.

You are given an integer N between 1 and 1998 (inclusive). Print the first three characters of the label of the N-th round of AtCoder Beginner Contest.

Constraints

1 \leq N \leq 1998

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the first three characters of the label of the N-th round of AtCoder Beginner Contest.

Sample Input 1

999

Sample Output 1

ABC

The 999-th round of AtCoder Beginner Contest is labeled as ABC999.

Sample Input 2

1000

Sample Output 2

ABD

The 1000-th round of AtCoder Beginner Contest is labeled as ABD001.

Sample Input 3

1481

Sample Output 3

ABD

The 1481-th round of AtCoder Beginner Contest is labeled as ABD482.

Solution in Python (3.4.3) (judge verdict: Accepted)

# -*- coding: utf-8 -*-
import sys
# ----------------------------------------------------------------
# Use Solve Function

def solve(lines):
    N = int(lines.pop(0))
    if N < 1000:
        print("ABC")
    else:
        print("ABD")

#
# main
#

# 1. use list instead of stdin

# contents = '''\
# 5\
# '''
# lines = contents.split('\n')

# 2. use stdin

lines = [x.strip() for x in sys.stdin.readlines()]

#
# solve !!
#
solve(lines)