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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include"bits/stdc++.h"
using namespace std;

//typedef
//------------------------------------------
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef vector<string> VS;
typedef pair<LL, LL> PLL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define PB push_back
#define MP make_pair
#define SORT(c) sort((c).begin(),(c).end())
#define COUT(x) cout<<x<<endl

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double PI = acos(-1.0);
const LL MOD = 1e9 + 7;
const ULL INF= 1e20;

//function
//--------------------------------------------
LL gcd(LL, LL);
bool isEven(LL);

int main() {
	int a;
	cin >> a;
	if (a <= 999)COUT("ABC");
	else COUT("ABD");
	return 0;
}

LL gcd(LL a, LL b) {
	if (a == 0 || b == 0)return 0;
	if (a < b)swap(a, b);
	if (a % b == 0)return b;
	else gcd(b, a % b);
}
bool isEven(LL a) {
	return !((bool)(a % 2));
}