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 C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int six = 1;
            int nine = 1;
            int count = 0;
            int N = int.Parse(Console.ReadLine());

            while (N >= 6)
            {
                if (N >= 9)
                {
                    while (N >= nine)
                    {
                        nine *= 9;
                    }

                    if (nine != 1)
                    {
                        nine /= 9;
                        count++;
                        N -= nine;
                        nine = 1;
                    }
                }

                if (N >= 6)
                {
                    while (N >= six)
                    {
                        six *= 6;
                    }

                    if (six != 1)
                    {
                        six /= 6;
                        count++;
                        N -= six;
                        six = 1;
                    }
                }
            }
            Console.WriteLine(count + N);
        }
    }
}