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: Runtime Error)

using System;

namespace ABC099_B
{
    class Program
    {
        static string Solve()
        {
            string input = Console.ReadLine();
            double towerWest = double.Parse(input.Split(' ')[0]);
            double towerEast = double.Parse(input.Split(' ')[1]);
            string snowfall = "";
            // 自然数の和　公式　1+2+3...+n=1/2n(n-1)　※(n+1)×n÷2 
            // 条件：積雪1m以上　高さ(1+2)以上なければ埋もれる
            if (towerWest >= 3)
            {
                // 積雪
                for (int i = 1; ; i++)
                {
                    double naturalWest = 3;     // 西側のタワー
                    double naturalEast = 3;     // 東側のタワー
                    // 塔
                    for (int n = 2; towerWest > (naturalWest - 1); n++)
                    {
                        naturalWest = (n + 1) * n / 2;
                        naturalEast = ((n + 1) + 1) * (n + 1) / 2;
                        if (((naturalWest - i) == towerWest) && ((naturalEast - i) == towerEast))
                        {
                            snowfall = i.ToString();
                            return snowfall;
                        }
                        else
                        {                        
                            n++;
                        }
                    }
                }
            }
            return snowfall;
        }
        static void Main(string[] args)
        {
            Console.WriteLine(Solve());
        }
    }
}