#include using namespace std; using int64 = long long; // const int mod = 1e9 + 7; const int mod = 998244353; const int64 infll = (1LL << 62) - 1; const int inf = (1 << 30) - 1; struct IoSetup { IoSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(10); cerr << fixed << setprecision(10); } } iosetup; template< typename T1, typename T2 > ostream &operator<<(ostream &os, const pair< T1, T2 > &p) { os << p.first << " " << p.second; return os; } template< typename T1, typename T2 > istream &operator>>(istream &is, pair< T1, T2 > &p) { is >> p.first >> p.second; return is; } template< typename T > ostream &operator<<(ostream &os, const vector< T > &v) { for(int i = 0; i < (int) v.size(); i++) { os << v[i] << (i + 1 != v.size() ? " " : ""); } return os; } template< typename T > istream &operator>>(istream &is, vector< T > &v) { for(T &in : v) is >> in; return is; } template< typename T1, typename T2 > inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); } template< typename T1, typename T2 > inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); } template< typename T = int64 > vector< T > make_v(size_t a) { return vector< T >(a); } template< typename T, typename... Ts > auto make_v(size_t a, Ts... ts) { return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...)); } template< typename T, typename V > typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) { t = v; } template< typename T, typename V > typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) { for(auto &e : t) fill_v(e, v); } template< typename F > struct FixPoint : F { FixPoint(F &&f) : F(forward< F >(f)) {} template< typename... Args > decltype(auto) operator()(Args &&... args) const { return F::operator()(*this, forward< Args >(args)...); } }; template< typename F > inline decltype(auto) MFP(F &&f) { return FixPoint< F >{forward< F >(f)}; } template< typename T = int > struct Edge { int from, to; T cost; int idx; Edge() = default; Edge(int from, int to, T cost = 1, int idx = -1) : from(from), to(to), cost(cost), idx(idx) {} operator int() const { return to; } }; template< typename T = int > struct Graph { vector< vector< Edge< T > > > g; int es; Graph() = default; explicit Graph(int n) : g(n), es(0) {} size_t size() const { return g.size(); } void add_directed_edge(int from, int to, T cost = 1) { g[from].emplace_back(from, to, cost, es++); } void add_edge(int from, int to, T cost = 1) { g[from].emplace_back(from, to, cost, es); g[to].emplace_back(to, from, cost, es++); } void read(int M, int padding = -1, bool weighted = false, bool directed = false) { for(int i = 0; i < M; i++) { int a, b; cin >> a >> b; a += padding; b += padding; T c = T(1); if(weighted) cin >> c; if(directed) add_directed_edge(a, b, c); else add_edge(a, b, c); } } }; template< typename T = int > using Edges = vector< Edge< T > >; /** * @brief Cycle Detection(閉路検出) * @docs docs/cycle-detection.md */ template< typename T = int > struct CycleDetection : Graph< T > { using Graph< T >::Graph; using Graph< T >::g; vector< int > used; Edges< T > pre, cyrcle; bool dfs(int idx) { used[idx] = 1; for(auto &e : g[idx]) { if(used[e] == 0) { pre[e] = e; if(dfs(e)) return true; } else if(used[e] == 1) { int cur = idx; while(cur != e) { cyrcle.emplace_back(pre[cur]); cur = pre[cur].from; } cyrcle.emplace_back(e); return true; } } used[idx] = 2; return false; } Edges< T > build() { used.assign(g.size(), 0); pre.resize(g.size()); for(int i = 0; i < (int) g.size(); i++) { if(used[i] == 0 && dfs(i)) { reverse(begin(cyrcle), end(cyrcle)); return cyrcle; } } return {}; } }; int main() { int H, W, N; cin >> H >> W >> N; vector< int > Y(N), X(N); for(int i = 0; i < N; i++) { cin >> Y[i] >> X[i]; --Y[i], --X[i]; } vector< vector< int > > ys(H), xs(W); for(int i = 0; i < N; i++) { ys[Y[i]].emplace_back(i); xs[X[i]].emplace_back(i); } CycleDetection<> g(N * 8); for(int i = 0; i < N; i++) { g.add_directed_edge(i, i + N); g.add_directed_edge(i + N + N, i + N + N + N); } int N4 = N + N + N + N; int N5 = N4 + N; int N6 = N5 + N; int N7 = N6 + N; for(int i = 0; i < W; i++) { for(int j = 1; j < xs[i].size(); j++) { g.add_directed_edge(xs[i][j - 1] + N4, xs[i][j] + N4); g.add_directed_edge(xs[i][j] + N5, xs[i][j - 1] + N5); } for(int j = 0; j < xs[i].size(); j++) { g.add_directed_edge(xs[i][j] + N4, xs[i][j] + N + N); g.add_directed_edge(xs[i][j] + N5, xs[i][j] + N + N); if(j > 0) { g.add_directed_edge(xs[i][j] + N, xs[i][j - 1] + N5); } if(j + 1 < xs[i].size()) { g.add_directed_edge(xs[i][j] + N, xs[i][j + 1] + N4); } } } for(int i = 0; i < H; i++) { for(int j = 1; j < ys[i].size(); j++) { g.add_directed_edge(ys[i][j - 1] + N6, ys[i][j] + N6); g.add_directed_edge(ys[i][j] + N7, ys[i][j - 1] + N7); } for(int j = 0; j < ys[i].size(); j++) { g.add_directed_edge(ys[i][j] + N6, ys[i][j]); g.add_directed_edge(ys[i][j] + N7, ys[i][j]); if(j > 0) { g.add_directed_edge(ys[i][j] + N + N + N, ys[i][j - 1] + N7); } if(j + 1 < ys[i].size()) { g.add_directed_edge(ys[i][j] + N + N + N, ys[i][j + 1] + N6); } } } g.build(); vector< int > ret; for(auto &p : g.cyrcle) { if(0 <= p.from and p.from < N) { ret.emplace_back(p.from + 1); } else if(N + N <= p.from and p.from < N + N + N) { ret.emplace_back(p.from - N - N + 1); } } if(ret.empty()) { throw 0; cout << "-1\n"; } else { assert(ret.size() >= 4); cout << ret.size() << "\n"; cout << ret << "\n"; } }