output_description: Print the first three characters of the label of the N-th round of AtCoder
  Beginner Contest.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
attempt: #include <bits/stdc++.h>
  using namespace std;
  
  //container util
  //------------------------------------------
  #define ALL(a)  (a).begin(),(a).end()
  #define RALL(a) (a).rbegin(), (a).rend()
  #define PB push_back
  #define MP make_pair
  #define SZ(a) int((a).size())
  #define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
  #define EXIST(s,e) ((s).find(e)!=(s).end())
  #define SORT(c) sort((c).begin(),(c).end())
  //repetition
  //------------------------------------------
  #define FOR(i,a,b) for(int i=(a);i<(b);++i)
  #define REP(i,n)  FOR(i,0,n)
  //conversion
  //------------------------------------------
  inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
  template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
  
  //debug
  #define dump(x)  cerr << #x << " = " << (x) << endl;
  #define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
  //math
  //-------------------------------------------
  template<class T> inline T sqr(T x) {return x*x;}
  
  typedef long long LL;
  const LL mod=1000000007;
  #define MAX 10000
  #define NIL -1
  const double PI  = acos(-1.0);
  const int INF = 1e9 + 7;
  //-------------------------------------------
  
  int n;
  
  int main(){
  
      cin.tie(0);
      ios::sync_with_stdio(false);
  
      cin >> n;
  
      if( n > 999)cout <<"ABD"<< endl;
      else { cout <<"ABC"<< endl;}
  
      return 0;
  }
  
problem_description: 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.
samples: [{"input": "999", "output": "ABC\n    \n\nThe 999-th round of AtCoder Beginner Contest is labeled as `ABC999`.\n\n* * *"}, {"input": "1000", "output": "ABD\n    \n\nThe 1000-th round of AtCoder Beginner Contest is labeled as `ABD001`.\n\n* * *"}, {"input": "1481", "output": "ABD\n    \n\nThe 1481-th round of AtCoder Beginner Contest is labeled as `ABD482`."}]