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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"flag"
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"strconv"
	"strings"
)

func (p *problem) solve() {
	N := p.NextInt()
	if N < 1000 {
		p.Printf("ABC")
	} else {
		p.Printf("ABD")
	}
}

var (
	debug  bool
	output string
)

func main() {
	flag.BoolVar(&debug, "debug", false, "Print debug logs")
	flag.BoolVar(&debug, "d", false, "Print debug logs")
	flag.StringVar(&output, "output", "stdout", "Specify output target (Default: stdout)")
	flag.StringVar(&output, "o", "stdout", "Specify output target (Default: stdout)")
	flag.Parse()

	if debug {
		out := os.Stdout
		if output != "stdout" {
			logfile, err := os.OpenFile(output, os.O_CREATE|os.O_WRONLY, 0666)
			if err != nil {
				panic(err)
			}
			defer logfile.Close()
			out = logfile
		}
		log.SetOutput(out)
	} else {
		log.SetOutput(ioutil.Discard)
	}

	p := problem{
		in:  bufio.NewReader(os.Stdin),
		out: bufio.NewWriter(os.Stdout),
	}
	p.solve()

	if err := p.out.Flush(); err != nil {
		panic(err)
	}
}

type problem struct {
	in  *bufio.Reader
	out *bufio.Writer
}

func (p *problem) PrintInt(d int) {
	p.Printf("%d\n", d)
}

func (p *problem) Printf(format string, a ...interface{}) {
	if _, err := fmt.Fprintf(p.out, format, a...); err != nil {
		panic(err)
	}
}

func (p *problem) Printfln(format string, a ...interface{}) {
	if _, err := fmt.Fprintf(p.out, fmt.Sprintf("%v\n", format), a...); err != nil {
		panic(err)
	}
}

func (p *problem) NextInt() int {
	return stoi(p.Next())
}

func (p *problem) NextInts() []int {
	line := p.NextSSLine()
	ds := make([]int, len(line))
	for i, s := range line {
		ds[i] = stoi(s)
	}
	return ds
}

func (p *problem) NextSSLine() []string {
	return strings.Split(p.Next(), " ")
}

// Next reads line and return string.
func (p *problem) Next() string {
	var b []byte
	for {
		l, pre, err := p.in.ReadLine()
		if err != nil {
			panic(err)
		}
		b = append(b, l...)
		if !pre {
			break
		}
	}
	return string(b)
}

func stoi(s string) int {
	i, err := strconv.Atoi(s)
	if err != nil {
		fmt.Println(i)
		fmt.Println(err)
	}
	return i
}