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)

#[allow(unused_imports)]
use std::cmp::{min,max};
#[allow(unused_imports)]
use std::collections::BTreeMap;
#[allow(unused_imports)]
use std::ops::*;
#[allow(unused_imports)]
use std::collections::BinaryHeap;

fn main() {
    let n = read_i64();
    println!("{}", if n <= 999 { "ABC" } else { "ABD" });
}

#[allow(dead_code)]
fn read_line() -> String {
    let mut ret = String::new();
    std::io::stdin().read_line(&mut ret).ok();
    ret.pop();
    return ret;
}

#[allow(dead_code)]
fn read_i64() -> i64 {
    let ss = read_line();
    return ss.parse::<i64>().unwrap();
}

#[allow(dead_code)]
fn read_vec_i64() -> Vec<i64> {
    let mut res = vec![];
    let ss = read_line();
    for ts in ss.split_whitespace() {
        let x = ts.parse::<i64>().unwrap();
        res.push(x);
    }
    return res;
}

use std::fmt::Display;
#[allow(dead_code)]
fn write_vec<T: Display>(xs: &Vec<T>) {
    if xs.len() == 0 {
        println!("");
        return;
    }
    print!("{}", xs[0]);
    for i in 1..xs.len() {
        print!(" {}", xs[i]);
    }
    println!("");
}