#include <string>
#include <vector>
#include <list>
#include <cmath>
#include <algorithm>
#include <map>
#include <set>
#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

class TopBiologist {
  public:
    string findShortestNewSequence(string sequence);
};

string TopBiologist::findShortestNewSequence(string sequence)
{
  int l, n;
  string letters = "ACGT";
  int i, j, k, top;
  string totest;

  n = letters.size();

  /* Keep enumerating this for successively larger strings. */

  for (l = 1; true; l++) {

    /* Calculate n^l */

    top = 1;
    for (i = 0; i < l; i++) top *= n;

     /* Enumerate each string, and see if it is sequence.  If not, return it. */

    for (i = 0; i < top; i++) {
      j = i;
      totest.clear();
      for (k = 0; k < l; k++) {
        totest.push_back(letters[j%n]);
        j /= n;
      }
      if (sequence.find(totest) == string::npos) return totest;
    }
  }
  return "";
}