/* #region Head */ #include using namespace std; using ll = long long; using ull = unsigned long long; using ld = long double; using pll = pair; template using vc = vector; template using vvc = vc>; using vll = vc; using vvll = vvc; using vld = vc; using vvld = vvc; using vs = vc; using vvs = vvc; template using um = unordered_map; template using pq = priority_queue; template using pqa = priority_queue, greater>; #define REP(i, m, n) for (ll i = (m), i##_len = (ll)(n); i < i##_len; ++(i)) #define REPM(i, m, n) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; ++(i)) #define REPR(i, m, n) for (ll i = (m), i##_min = (ll)(n); i >= i##_min; --(i)) #define REPD(i, m, n, d) for (ll i = (m), i##_len = (ll)(n); i < i##_len; i += (d)) #define REPMD(i, m, n, d) for (ll i = (m), i##_max = (ll)(n); i <= i##_max; i += (d)) #define REPI(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++) #define ALL(x) begin(x), end(x) #define SIZE(x) ((ll)(x).size()) #define PERM(c) \ sort(ALL(c)); \ for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c))) #define UNIQ(v) v.erase(unique(ALL(v)), v.end()); #define endl '\n' #define sqrt sqrtl #define floor floorl #define log2 log2l constexpr ll INF = 1'010'000'000'000'000'017LL; constexpr ll MOD = 1'000'000'007LL; // 1e9 + 7 constexpr ld EPS = 1e-12; constexpr ld PI = 3.14159265358979323846; template istream &operator>>(istream &is, vc &vec) { // vector 入力 for (T &x : vec) is >> x; return is; } template ostream &operator<<(ostream &os, vc &vec) { // vector 出力 (for dump) os << "{"; REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "" : ", "); os << "}"; return os; } template ostream &operator>>(ostream &os, vc &vec) { // vector 出力 (inline) REP(i, 0, SIZE(vec)) os << vec[i] << (i == i_len - 1 ? "\n" : " "); return os; } template istream &operator>>(istream &is, pair &pair_var) { // pair 入力 is >> pair_var.first >> pair_var.second; return is; } template ostream &operator<<(ostream &os, pair &pair_var) { // pair 出力 os << "(" << pair_var.first << ", " << pair_var.second << ")"; return os; } // map, um, set 出力 template ostream &out_iter(ostream &os, T &map_var) { os << "{"; REPI(itr, map_var) { os << *itr; auto itrcp = itr; if (++itrcp != map_var.end()) os << ", "; } os << "}"; return os; } template ostream &operator<<(ostream &os, map &map_var) { return out_iter(os, map_var); } template ostream &operator<<(ostream &os, um &map_var) { return out_iter(os, map_var); } template ostream &operator<<(ostream &os, set &set_var) { return out_iter(os, set_var); } // dump #define DUMPOUT cerr void dump_func() { DUMPOUT << endl; } template void dump_func(Head &&head, Tail &&... tail) { DUMPOUT << head; if (sizeof...(Tail) > 0) DUMPOUT << ", "; dump_func(move(tail)...); } // chmax (更新「される」かもしれない値が前) template > bool chmax(T &xmax, const U &x, Comp comp = {}) { if (comp(xmax, x)) { xmax = x; return true; } return false; } // chmin (更新「される」かもしれない値が前) template > bool chmin(T &xmin, const U &x, Comp comp = {}) { if (comp(x, xmin)) { xmin = x; return true; } return false; } // ローカル用 #define DEBUG_ #ifdef DEBUG_ #define DEB #define dump(...) \ DUMPOUT << " " << string(#__VA_ARGS__) << ": " \ << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]" << endl \ << " ", \ dump_func(__VA_ARGS__) #else #define DEB if (false) #define dump(...) #endif struct AtCoderInitialize { static constexpr int IOS_PREC = 15; static constexpr bool AUTOFLUSH = false; AtCoderInitialize() { ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr); cout << fixed << setprecision(IOS_PREC); if (AUTOFLUSH) cout << unitbuf; } } ATCODER_INITIALIZE; string yes = "Yes", no = "No"; // string yes = "YES", no = "NO"; void yn(bool p) { cout << (p ? yes : no) << endl; } /* #endregion */ /* #region Graph */ // グラフ用テンプレ using Weight = ll; using Flow = ll; // エッジ(本来エッジは双方向だが,ここでは単方向で管理) struct Edge { ll src; // エッジ始点となる頂点 ll dst; // エッジ終点となる頂点 Weight weight; // 重み Flow cap; Edge() : src(0), dst(0), weight(0) {} Edge(ll src, ll dst, Weight weight) : src(src), dst(dst), weight(weight) {} }; using Node = vc; // 同じ頂点を始点とするエッジ集合 using Graph = vc; // graph[i] := 頂点 i を始点とするエッジ集合 using Array = vector; using Matrix = vector; // 双方向のエッジを追加する void add_edge(Graph &g, ll a, ll b, Weight w = 1) { g[a].emplace_back(a, b, w); g[b].emplace_back(b, a, w); } // 単方向のアークを追加する void add_arc(Graph &g, ll a, ll b, Weight w = 1) { g[a].emplace_back(a, b, w); } // Edge 標準出力 ostream &operator<<(ostream &os, Edge &edge) { os << "(" << edge.src << " -> " << edge.dst << ", " << edge.weight << ")"; return os; } /* #endregion */ bool has_cycle_undirected(Graph &graph) { ll n = SIZE(graph); vc visited(n, false); function dfs = [&](ll idx, ll par) { if (visited[idx]) return true; visited[idx] = true; for (Edge &edge : graph[idx]) if (edge.dst != par && dfs(edge.dst, idx)) return true; return false; }; REP(i, 0, n) if (!visited[i]) if (dfs(i, -1)) return true; return false; } // Problem void solve() { string ok = "possible", ng = "impossible"; ll w, h; cin >> w >> h; vvll m(h, vll(w)); cin >> m; Graph g(w * h); REP(i, 0, h) REP(j, 0, w) { ll id = w * i + j; if (i < h - 1 && m[i][j] == m[i + 1][j]) add_edge(g, id, id + w); // 下 if (j < w - 1 && m[i][j] == m[i][j + 1]) add_edge(g, id, id + 1); } cout << (has_cycle_undirected(g) ? ok : ng) << endl; } // entry point int main() { solve(); return 0; }