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: Wrong Answer)

def main():
    n = int(input())
    dp = [float('inf')]*(n+1)
    dp[0] = 0

    ls = [1, 6, 9]
    ks = [1]

    for l in ls[1:]:
        i = 1
        while True:
            if l ** i <= n:
                ks.append(l**i)
            else:
                break
            i += 1

    for k in ks:
        for i in range(len(dp)):
            if i+k >= len(dp):
                break
            if dp[i]+1 < dp[i+k]:
                dp[i+k] = dp[i]+1
    print(dp[n])

main()