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 pair<int,int> pii;
typedef long long int ll;

#define INF 1 << 30
#define REP(i,n) for(ll i=0; i<(int)(n); i++)
#define FOR(i,k,n) for(ll i=(k);i<(int)(n);i++)



vector<bool> gen_sosuu(vector<bool> sosuu){
  int size = sosuu.size();
  REP(i,size){
    sosuu[i] = true;
  }
  sosuu[0] = false;
  sosuu[1] = false;
  sosuu[2] = true;
  FOR(i,2,sqrt(size)+1){
    if(sosuu[i] == false) continue;
    for(int j = 2; i*j<size;j++){
      sosuu[i*j] =false;
    }
  }
  return sosuu;
}

int n;

int main(){
  cin >> n;
  if(n>=1000){
    cout << "ABD" <<endl;
  }else{
    cout <<"ABC"<<endl;
  }
}