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 Rust (1.15.1) (judge verdict: Accepted)

use std::io::BufRead;

fn main() {
    let stdin = std::io::stdin();
    let n = stdin.lock().gets().parse::<u32>().unwrap();
    let gen = (n - 1) / 999;
    if gen < 24 {
        println!("AB{}", (b'C' + gen as u8) as char);
    } else {
        panic!("Too future");
    }
}

trait Gets { fn gets(&mut self) -> String; }
impl<T: std::io::BufRead> Gets for T {
    fn gets(&mut self) -> String { self.lines().next().unwrap().unwrap() } }