// compile: make data
// run: ./data < data.in
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#include <debug/codeforces.h>
#define debug(x...) _debug_print(#x, x);
#define Debug(x...) _debug_print_format(#x, x);
std::ifstream terminal("/dev/tty");
#define PP cerr<<"\033[1;30mpause...\e[0m",terminal.ignore();
#else
#define debug(x...)
#define Debug(x...)
#define PP
#endif
template<typename...Args> void print_(Args...args){((cout<<args<<" "),...)<<endl;}
#define rep(i,a,b) for(int i=(a);i<(int)(b);++i)
#define sz(v) ((int)(v).size())
#define print(...) print_(__VA_ARGS__);
#define FIND(a, x) ((find(a.begin(),a.end(),(x))!=a.end())?1:0)
#define cmin(x,...) x=min({(x),__VA_ARGS__})
#define cmax(x,...) x=max({(x),__VA_ARGS__})
#define INTMAX (int)(9223372036854775807)
#define INF (int)(1152921504606846976)
#define NaN (int)(0x8b88e1d0595d51d1)
#define double long double
#define int long long
#define uint unsigned long long
#define MAXN 200010
 
struct graph {
    struct node {
        int u, v, w;
        int idx;
        bool operator<(const node &other) const {return w < other.w; }
        bool operator==(const node &other) const {return v == other.v && w == other.w; }
    };
    vector<vector<node>> e;
    vector<vector<int>> invid;
    node &inv(node &edge) {return e[edge.v][invid[edge.u][edge.idx]]; }
 
    int n;
    bool directed;
    graph(int V, bool D = 0) {
        n = V;
        e.resize(n), invid.resize(n);
        directed = D;
    }
    void add_edge(int u, int v, int w = 1, int wb = 1) {
        e[u].push_back(node{u, v, w, sz(e[u])});
        if (!directed) {
            e[v].push_back(node{v, u, wb, sz(e[v])});
            invid[u].push_back(sz(e[v]) - 1);
            invid[v].push_back(sz(e[u]) - 1);
        }
    }
    void graphviz_dump(int plus = 0, string filename = "graph.dot") {
        ofstream gf; gf.open(filename);
        gf << (directed ? "digraph" : "graph") << " {\n";
        gf << "    "; rep(i, 0, n) gf << i+plus << " ;"[i==n-1]; gf << endl;
        string notation = directed ? " -> " : " -- ";
        bool weighted = 0;
        for (auto es: e) for (auto edge: es) if (edge.w != 1) weighted = 1;
        rep(u, 0, n) {
            for (auto edge: e[u]) {
                int v = edge.v, w = edge.w;
                if (!directed && u > v) continue;
                gf << "    " << u+plus << notation << v+plus << (weighted ? " ;\n" : ";\n");
            }
        }
        gf << "}\n";
    }
    pair<vector<int>, vector<int>> bfs(int s) {
        queue<int> q; q.push(s);
        vector<int> vis(n, 0); vis[s] = 1;
        vector<int> dis(n, INF); dis[s] = 0;
        vector<int> pre(n); pre[s] = -1;
        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (auto edge: e[u]) {
                int v = edge.v;
                if (vis[v]) continue;
                vis[v] = 1;
                dis[v] = dis[u] + 1;
                pre[v] = u;
                q.push(v);
            }
        }
        return {dis, pre};
    }
};
 
int32_t main() {
    ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
 
    string s; cin >> s;
    vector<int> A, C, G, T;
    rep(i, 0, sz(s)) {
        if (s[i] == 'A') A.push_back(i);
        else if (s[i] == 'C') C.push_back(i);
        else if (s[i] == 'G') G.push_back(i);
        else if (s[i] == 'T') T.push_back(i);
    }
    int Aptr = 0, Cptr = 0, Gptr = 0, Tptr = 0;
    graph g(sz(s), 1);
    rep(i, 0, sz(s)) {
        if (s[i] == 'A') {
            if (Aptr+1 < sz(A)) g.add_edge(A[Aptr], A[Aptr+1]);
            if (Cptr < sz(C)) g.add_edge(A[Aptr], C[Cptr]);
            if (Gptr < sz(G)) g.add_edge(A[Aptr], G[Gptr]);
            if (Tptr < sz(T)) g.add_edge(A[Aptr], T[Tptr]);
            ++Aptr;
        }
        if (s[i] == 'C') {
            if (Aptr < sz(A)) g.add_edge(C[Cptr], A[Aptr]);
            if (Cptr+1 < sz(C)) g.add_edge(C[Cptr], C[Cptr+1]);
            if (Gptr < sz(G)) g.add_edge(C[Cptr], G[Gptr]);
            if (Tptr < sz(T)) g.add_edge(C[Cptr], T[Tptr]);
            ++Cptr;
        }
        if (s[i] == 'G') {
            if (Aptr < sz(A)) g.add_edge(G[Gptr], A[Aptr]);
            if (Cptr < sz(C)) g.add_edge(G[Gptr], C[Cptr]);
            if (Gptr+1 < sz(G)) g.add_edge(G[Gptr], G[Gptr+1]);
            if (Tptr < sz(T)) g.add_edge(G[Gptr], T[Tptr]);
            ++Gptr;
        }
        if (s[i] == 'T') {
            if (Aptr < sz(A)) g.add_edge(T[Tptr], A[Aptr]);
            if (Cptr < sz(C)) g.add_edge(T[Tptr], C[Cptr]);
            if (Gptr < sz(G)) g.add_edge(T[Tptr], G[Gptr]);
            if (Tptr+1 < sz(T)) g.add_edge(T[Tptr], T[Tptr+1]);
            ++Tptr;
        }
    }
    // g.graphviz_dump();
    if (A.empty()) {cout << "A" << endl; return 0; }
    if (C.empty()) {cout << "C" << endl; return 0; }
    if (G.empty()) {cout << "G" << endl; return 0; }
    if (T.empty()) {cout << "T" << endl; return 0; }
    auto [Adis, Apre] = g.bfs(A[0]);
    auto [Cdis, Cpre] = g.bfs(C[0]);
    auto [Gdis, Gpre] = g.bfs(G[0]);
    auto [Tdis, Tpre] = g.bfs(T[0]);
    char start = '\0';
    int mindis = INF, end = -1;
    rep(i, 0, g.n) {
        if (g.e[i].size() == 4) continue;
        if (Adis[i] < mindis) mindis = Adis[i], start = 'A', end = i;
        if (Cdis[i] < mindis) mindis = Cdis[i], start = 'C', end = i;
        if (Gdis[i] < mindis) mindis = Gdis[i], start = 'G', end = i;
        if (Tdis[i] < mindis) mindis = Tdis[i], start = 'T', end = i;
    }
    vector<int> *pre;
    if (start == 'A') pre = &Apre;
    if (start == 'C') pre = &Cpre;
    if (start == 'G') pre = &Gpre;
    if (start == 'T') pre = &Tpre;
    string res = "";
    vector<int> hs(26, 0);
    for (auto c: g.e[end]) ++hs[s[c.v]-'A'];
    if (hs['A'-'A'] == 0) res = 'A';
    else if (hs['C'-'A'] == 0) res = 'C';
    else if (hs['G'-'A'] == 0) res = 'G';
    else if (hs['T'-'A'] == 0) res = 'T';
    // g.graphviz_dump();
    while (end != -1) {
        res += s[end];
        end = (*pre)[end];
    }
    reverse(res.begin(), res.end());
    cout << res << endl;
 
    return 0;
}