#define LOCAL #include using namespace std; #pragma region Macros typedef long long ll; typedef __int128_t i128; typedef unsigned int uint; typedef unsigned long long ull; #define ALL(x) (x).begin(), (x).end() template istream& operator>>(istream& is, vector& v) { for (T& x : v) is >> x; return is; } template ostream& operator<<(ostream& os, const vector& v) { for (int i = 0; i < (int)v.size(); i++) { os << v[i] << (i + 1 == (int)v.size() ? "" : " "); } return os; } template ostream& operator<<(ostream& os, const pair& p) { os << '(' << p.first << ',' << p.second << ')'; return os; } template ostream& operator<<(ostream& os, const tuple& t) { os << '(' << get<0>(t) << ',' << get<1>(t) << ',' << get<2>(t) << ')'; return os; } template ostream& operator<<(ostream& os, const tuple& t) { os << '(' << get<0>(t) << ',' << get<1>(t) << ',' << get<2>(t) << ',' << get<3>(t) << ')'; return os; } template ostream& operator<<(ostream& os, const map& m) { os << '{'; for (auto itr = m.begin(); itr != m.end();) { os << '(' << itr->first << ',' << itr->second << ')'; if (++itr != m.end()) os << ','; } os << '}'; return os; } template ostream& operator<<(ostream& os, const unordered_map& m) { os << '{'; for (auto itr = m.begin(); itr != m.end();) { os << '(' << itr->first << ',' << itr->second << ')'; if (++itr != m.end()) os << ','; } os << '}'; return os; } template ostream& operator<<(ostream& os, const set& s) { os << '{'; for (auto itr = s.begin(); itr != s.end();) { os << *itr; if (++itr != s.end()) os << ','; } os << '}'; return os; } template ostream& operator<<(ostream& os, const multiset& s) { os << '{'; for (auto itr = s.begin(); itr != s.end();) { os << *itr; if (++itr != s.end()) os << ','; } os << '}'; return os; } template ostream& operator<<(ostream& os, const unordered_set& s) { os << '{'; for (auto itr = s.begin(); itr != s.end();) { os << *itr; if (++itr != s.end()) os << ','; } os << '}'; return os; } template ostream& operator<<(ostream& os, const deque& v) { for (int i = 0; i < (int)v.size(); i++) { os << v[i] << (i + 1 == (int)v.size() ? "" : " "); } return os; } void debug_out() { cerr << '\n'; } template void debug_out(Head&& head, Tail&&... tail) { cerr << head; if (sizeof...(Tail) > 0) cerr << ", "; debug_out(move(tail)...); } #ifdef LOCAL #define debug(...) \ cerr << " "; \ cerr << #__VA_ARGS__ << " :[" << __LINE__ << ":" << __FUNCTION__ << "]" << '\n'; \ cerr << " "; \ debug_out(__VA_ARGS__) #else #define debug(...) 42 #endif template T gcd(T x, T y) { return y != 0 ? gcd(y, x % y) : x; } template T lcm(T x, T y) { return x / gcd(x, y) * y; } template inline bool chmin(T1& a, T2 b) { if (a > b) { a = b; return true; } return false; } template inline bool chmax(T1& a, T2 b) { if (a < b) { a = b; return true; } return false; } #pragma endregion /** * @brief Strongly Connected Components * @docs docs/graph/StronglyConnectedComponents.md */ struct StronglyConnectedComponents { vector> G, rG, C, T; vector vs, cmp, used; StronglyConnectedComponents(int n) : G(n), rG(n), cmp(n), used(n) {} void add_edge(int u, int v) { G[u].emplace_back(v); rG[v].emplace_back(u); } void dfs(int v) { used[v] = 1; for (int u : G[v]) if (!used[u]) dfs(u); vs.emplace_back(v); } void rdfs(int v, int k) { used[v] = 1; cmp[v] = k; C[k].emplace_back(v); for (int u : rG[v]) if (!used[u]) rdfs(u, k); } int build() { int n = G.size(); for (int i = 0; i < n; i++) if (!used[i]) dfs(i); fill(used.begin(), used.end(), 0); int k = 0; for (int i = n - 1; i >= 0; --i) { if (!used[vs[i]]) { C.emplace_back(), T.emplace_back(); rdfs(vs[i], k++); } } for (int v = 0; v < n; v++) { for (int u : G[v]) { if (cmp[v] != cmp[u]) { T[cmp[v]].emplace_back(cmp[u]); } } } for (int i = 0; i < k; i++) { sort(T[i].begin(), T[i].end()); T[i].erase(unique(T[i].begin(), T[i].end()), T[i].end()); } return k; } int operator[](int i) const { return cmp[i]; } }; const int INF = 1e9; const long long IINF = 1e18; const int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1}; const char dir[4] = {'D', 'R', 'U', 'L'}; const long long MOD = 1000000007; // const long long MOD = 998244353; int main() { cin.tie(0); ios::sync_with_stdio(false); int N, M; cin >> N >> M; StronglyConnectedComponents SCC(N); for (int i = 0; i < M; i++) { int U, V; cin >> U >> V; SCC.add_edge(--U, --V); } int k = SCC.build(); vector> ans; auto &T = SCC.T, &C = SCC.C; set s; for (int i = 0; i < k; i++) { int sz = C[i].size(); if (sz > 1) { for (int j = 0; j + 1 < sz; j++) { ans.emplace_back(C[i][j], C[i][j + 1]); } } if (s.count(i)) s.erase(i); for (int& x : T[i]) s.emplace(x); if (!s.empty()) ans.emplace_back(C[i].back(), C[i + 1][0]); } cout << ans.size() << '\n'; for (auto& p : ans) cout << p.first + 1 << ' ' << p.second + 1 << '\n'; return 0; }