#include <bits/stdc++.h>
#ifndef LOCAL
#include "combo.h"
#endif
using namespace std;
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define all(a) (a).begin(), (a).end()
#define Time (clock() * 1.0 / CLOCKS_PER_SEC)
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
using ld = long double;
template<typename T1, typename T2> bool chkmin(T1& x, T2 y) {
return y < x ? (x = y, true) : false;
}
template<typename T1, typename T2> bool chkmax(T1& x, T2 y) {
return y > x ? (x = y, true) : false;
}
void debug_out() {
cerr << endl;
}
template<typename T1, typename... T2> void debug_out(T1 A, T2... B) {
cerr << ' ' << A;
debug_out(B...);
}
template<typename T> void mdebug_out(T* a, int n) {
for (int i = 0; i < n; ++i)
cerr << a[i] << ' ';
cerr << endl;
}
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define mdebug(a, n) cerr << #a << ": ", mdebug_out(a, n)
#else
#define debug(...) 1337
#define mdebug(a, n) 1337
#endif
template<typename T> ostream& operator << (ostream& stream, const vector<T>& v) {
for (auto& x : v)
stream << x << ' ';
return stream;
}
template<typename T1, typename T2> ostream& operator << (ostream& stream, const pair<T1, T2>& p) {
return stream << p.first << ' ' << p.second;
}
#ifdef LOCAL
mt19937 rng(228);
int n;
string s;
int press(string p) {
int mx = 0;
for (int i = 0; i < p.size(); ++i) {
int lcp = 0;
while (lcp < n && i + lcp < p.size() && s[lcp] == p[i + lcp])
++lcp;
chkmax(mx, lcp);
}
return mx;
}
#endif
const string ch[4] = {"A", "B", "X", "Y"};
// int Press(vector<int> v) {
// string s(v.size());
// for (int i = 0; i < v.size(); ++i)
// s[i] = ch[v[i]];
// returu press(s);
// }
string guess_sequence(int n) {
mt19937 rng(time(0));
vector<int> ord_z = {0, 1, 2, 3};
shuffle(all(ord_z), rng);
int zi = 0;
while (zi < 3 && press(ch[ord_z[zi]]) == 0)
++zi;
int z = ord_z[zi];
if (n == 1) {
return ch[z];
}
vector<string> rem;
for (int i = 0; i < 4; ++i)
if (i != z)
rem.push_back(ch[i]);
string cur = ch[z];
for (int i = 1; i + 1 < n; ++i) {
string q = cur + rem[0] + rem[0] + cur + rem[0] + rem[1] + cur + rem[0] + rem[2] + cur + rem[1];
int res = press(q);
if (res == i) {
cur += rem[2];
} else if (res == i + 1) {
cur += rem[1];
} else {
cur += rem[0];
}
}
vector<int> ord_l = {0, 1, 2};
shuffle(all(ord_l), rng);
int li = 0;
while (li < 2 && press(cur + rem[ord_l[li]]) < n)
++li;
int l = ord_l[li];
cur += rem[l];
return cur;
}
#ifdef LOCAL
int32_t main() {
#ifdef LOCAL
freopen("in", "r", stdin);
#endif
ios::sync_with_stdio(0);
cin.tie(0);
for (int test = 1; ; ++test) {
n = 10;
int z = rng() % 4;
s = ch[z];
vector<string> rem;
for (int i = 0; i < 4; ++i)
if (i != z)
rem.push_back(ch[i]);
for (int i = 1; i < n; ++i)
s += rem[rng() % 3];
if (guess_sequence(n) != s) {
debug(s);
exit(0);
}
if (test % 10000 == 0) debug(test);
}/**/
cin >> s;
n = s.size();
cout << guess_sequence(n) << '\n';
return 0;
}
#endif
