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)

macro_rules! input {
    // ?
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };

    // stdinの生成 値の読み込み開始
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    // ?
    ($next:expr) => {};
    // ?
    ($next:expr, ) => {};

    // 変数名:型の読み込みの繰り返し
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    // 値を読み込むベース 型,の繰り返し
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    // 配列の読み込み
    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    // 文字列Vec<char>の読み込み
    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    // usize1?の読み込み
    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    // 指定した型で読み込み
    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}
fn main() {
    input! {
        num: usize,
    };

    println!("{}", solve(num));
}

fn solve(num: usize) -> &'static str {
    if num < 1000 {
        return "ABC";
    } else {
        return "ABD";
    }
}
macro_rules! tests {
    ($($test_name:ident:$name:ident($( $args:expr ),*) => $expected:expr,)*) => {
        mod tests {
            $(
                #[test]
                fn $test_name() {
                    assert_eq!($expected,super::$name($($args),*));
                }
            )*
        }
    }
}


tests! {
    //テスト名:[引数]=>expected
    test1:solve(1000) => "ABD",
    test2:solve(999) => "ABC",
    test3:solve(1481) => "ABD",
}