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: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
class P
{
    static void Main()
    {
        int n = ReadInt();
        bool f = n < 1000;
        WriteLine(f ? "ABC" : "ABD");
    }
    //以下標準出入力用のメソッド
    [System.Runtime.InteropServices.DllImport("msvcrt.dll")]
    private static extern int scanf(string format, out long l);
    public static long ReadLong()
    {
        long l;
        scanf("%ld", out l);
        return l;
    }
    public static int ReadInt()
    {
        return (int)ReadLong();
    }
    public static int[] ReadInts(int n)
    {
        int[] nums = new int[n];
        for (int i = 0; i < n; i++)
            nums[i] = ReadInt();
        return nums;
    }
    public static long[] ReadLongs(int n)
    {
        long[] nums = new long[n];
        for (int i = 0; i < n; i++)
            nums[i] = ReadLong();
        return nums;
    }
    [System.Runtime.InteropServices.DllImport("msvcrt.dll")]
    private static extern int scanf(string format, out double l);
    public static double ReadDouble()
    {
        double d;
        scanf("%lf", out d);
        return d;
    }
    [System.Runtime.InteropServices.DllImport("msvcrt.dll")]
    private static extern int scanf(string format, out char c);
    public static char ReadChar()
    {
        char c;
        while (true)
        {
            scanf("%c", out c);
            if (c != '\n')
                break;
        }
        return c;
    }
    public static string ReadLine()
    {
        StringBuilder sb = new StringBuilder();
        char c;
        do
        {
            scanf("%c", out c);
            if (c != '\n')
                sb.Append(c);
        } while (c != '\n');
        return sb.ToString();
    }
    [System.Runtime.InteropServices.DllImport("msvcrt.dll")]
    private static extern void printf(string s);
    public static void Write(string s) { printf(s); }
    public static void Write(char c) { printf(c.ToString()); }
    public static void Write(long n) { printf(n.ToString()); }
    public static void Write(int n) { printf(n.ToString()); }
    public static void Write(double d) { printf(d.ToString()); }
    public static void WriteLine() { printf("\n"); }
    public static void WriteLine(string s) { printf(s + "\n"); }
    public static void WriteLine(char c) { WriteLine(c.ToString()); }
    public static void WriteLine(long n) { WriteLine(n.ToString()); }
    public static void WriteLine(int n) { WriteLine(n.ToString()); }
    public static void WriteLine(double d) { WriteLine(d.ToString()); }
    public static void WriteLine(string[] s)
    {
        for (int i = 0; i < s.Length - 1; i++)
            Write(s[i] + " ");
        WriteLine(s[s.Length - 1]);
    }
    public static void WriteLine(char[] c)
    {
        for (int i = 0; i < c.Length - 1; i++)
            Write(c[i] + " ");
        WriteLine(c[c.Length - 1]);
    }
    public static void WriteLine(long[] n)
    {
        for (int i = 0; i < n.Length - 1; i++)
            Write(n[i] + " ");
        WriteLine(n[n.Length - 1]);
    }
    public static void WriteLine<T>(ICollection<T> ic)
    {
        foreach (var i in ic)
            Write(i + " ");
        WriteLine("\b");
    }
    public static void WriteLine(int[] n)
    {
        for (int i = 0; i < n.Length - 1; i++)
            Write(n[i] + " ");
        WriteLine(n[n.Length - 1]);
    }
    public static void WriteLine(double[] d)
    {
        for (int i = 0; i < d.Length - 1; i++)
            Write(d[i] + " ");
        WriteLine(d[d.Length - 1]);
    }
    public static void WriteLines(string[] s) { foreach (var i in s) WriteLine(i); }
    public static void WriteLines(char[] c) { foreach (var i in c) WriteLine(i); }
    public static void WriteLines(long[] c) { foreach (var i in c) WriteLine(i); }
    public static void WriteLines(int[] c) { foreach (var i in c) WriteLine(i); }
    public static void WriteLines(double[] d) { foreach (var i in d) WriteLine(i); }
    public static void WriteLines<T>(ICollection<T> ic) { WriteLines(ic.ToArray()); }
}