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 Ruby (2.3.3) (judge verdict: Accepted)

#!/usr/bin/env ruby

# Generated by AtcoderGreedy on 2018-06-10
# ABC099 A

# my lib

module Enumerable
  # work just like String#split,
  # except this method does not use regex match
  # but check whether yield succeeds or not.
  def split
    current = nil
    ret = []
    each do |elem|
      if yield elem
        if current
          ret << current
          current = nil
        end
      elsif current
        current << elem
      else
        current = [elem]
      end
    end
    ret << current if current
    ret
  end
end

class Array
  # truncate tail, opposite of drop
  def clip(amount = 1)
    if amount < 0
      raise ArgumentError, "amount == #{amount}, cannot be negative!"
    end
    take_amount = size - amount
    take_amount < 0 ? [] : take(take_amount)
  end
end

def n_choose_k(n, k)
  return 0 unless (0..n).cover? k
  result = 1
  1.upto(k) do |d|
    result *= n
    result /= d
    n -= 1
  end
  result
end

# end of lib

# Your code here

N = gets.chomp.to_i

if N >= 1000
  puts 'ABD'
else
  puts 'ABC'
end