// >>> TEMPLATES #include using namespace std; using ll = long long; using ld = long double; using i32 = int32_t; using i64 = int64_t; using u32 = uint32_t; using u64 = uint64_t; #define int ll #define rep(i, n) for (int i = 0; i < (int)(n); i++) #define rep1(i, n) for (int i = 1; i <= (int)(n); i++) #define repR(i, n) for (int i = (int)(n)-1; i >= 0; i--) #define rep1R(i, n) for (int i = (int)(n); i >= 1; i--) #define loop(i, a, B) for (int i = a; i B; i++) #define loopR(i, a, B) for (int i = a; i B; i--) #define all(x) begin(x), end(x) #define allR(x) rbegin(x), rend(x) #define rng(x, l, r) begin(x) + (l), begin(x) + (r) #define pb push_back #define eb emplace_back #define fst first #define snd second template constexpr auto mp(A &&a, B &&b) { return make_pair(forward(a), forward(b)); } template constexpr auto mt(T&&... x) { return make_tuple(forward(x)...); } template auto constexpr inf_ = numeric_limits::max()/2-1; auto constexpr INF32 = inf_; auto constexpr INF64 = inf_; auto constexpr INF = inf_; #ifdef LOCAL #include "debug.hpp" #else #define dump(...) (void)(0) #define say(x) (void)(0) #define debug if (0) #endif template struct pque : priority_queue, Comp> { vector &data() { return this->c; } void clear() { this->c.clear(); } }; template using pque_max = pque>; template using pque_min = pque>; template ::value, int> = 0> ostream& operator<<(ostream& os, T const& a) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; } template ::value, int> = 0> ostream& operator<<(ostream& os, const T (&a)[N]) { bool f = true; for (auto const& x : a) os << (f ? "" : " ") << x, f = false; return os; } template ())), class = typename enable_if::value>::type> istream& operator>>(istream& is, T &a) { for (auto& x : a) is >> x; return is; } template ostream& operator<<(ostream& os, pair const& p) { return os << p.first << " " << p.second; } template istream& operator>>(istream& is, pair& p) { return is >> p.first >> p.second; } struct IOSetup { IOSetup() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); } } iosetup; template struct FixPoint : private F { constexpr FixPoint(F&& f) : F(forward(f)) {} template constexpr auto operator()(T&&... x) const { return F::operator()(*this, forward(x)...); } }; struct MakeFixPoint { template constexpr auto operator|(F&& f) const { return FixPoint(forward(f)); } }; #define MFP MakeFixPoint()| #define def(name, ...) auto name = MFP [&](auto &&name, __VA_ARGS__) template struct vec_impl { using type = vector::type>; template static type make_v(size_t n, U&&... x) { return type(n, vec_impl::make_v(forward(x)...)); } }; template struct vec_impl { using type = T; static type make_v(T const& x = {}) { return x; } }; template using vec = typename vec_impl::type; template auto make_v(Args&&... args) { return vec_impl::make_v(forward(args)...); } template void quit(T const& x) { cout << x << endl; exit(0); } template constexpr bool chmin(T& x, U const& y) { if (x > (T)y) { x = (T)y; return true; } return false; } template constexpr bool chmax(T& x, U const& y) { if (x < (T)y) { x = (T)y; return true; } return false; } template constexpr auto sumof(It b, It e) { return accumulate(b, e, typename iterator_traits::value_type{}); } template int sz(T const& x) { return x.size(); } template int lbd(C const& v, T const& x) { return lower_bound(begin(v), end(v), x)-begin(v); } template int ubd(C const& v, T const& x) { return upper_bound(begin(v), end(v), x)-begin(v); } constexpr int64_t mod(int64_t x, int64_t m) { assert(m > 0); return (x %= m) < 0 ? x+m : x; } constexpr int64_t div_floor(int64_t x, int64_t y) { assert(y != 0); return x/y - ((x^y) < 0 and x%y); } constexpr int64_t div_ceil(int64_t x, int64_t y) { assert(y != 0); return x/y + ((x^y) > 0 and x%y); } constexpr int dx[] = { 1, 0, -1, 0, 1, -1, -1, 1 }; constexpr int dy[] = { 0, 1, 0, -1, 1, 1, -1, -1 }; constexpr int popcnt(ll x) { return __builtin_popcountll(x); } mt19937_64 seed_{random_device{}()}; template Int rand(Int a, Int b) { return uniform_int_distribution(a, b)(seed_); } i64 irand(i64 a, i64 b) { return rand(a, b); } // [a, b] u64 urand(u64 a, u64 b) { return rand(a, b); } // template void shuffle(It l, It r) { shuffle(l, r, seed_); } template vector &operator--(vector &v) { for (T &x : v) --x; return v; } template vector &operator++(vector &v) { for (T &x : v) ++x; return v; } // <<< // >>> union find struct UnionFind { int n, sz; // id : 0...n-1 vector par; UnionFind(int n = 0) : n(n), sz(n), par(n, -1) { } void clear() { rep (i, n) par[i] = -1; sz = n; } int root(int x) { assert(0 <= x); assert(x < n); return par[x] < 0 ? x : par[x] = root(par[x]); } bool unite(int x, int y) { x = root(x), y = root(y); if (x == y) return false; sz--; if (par[x] < par[y]) swap(x, y); par[y] += par[x]; par[x] = y; return true; } bool same(int x, int y) { return root(x) == root(y); } int size(int x) { return -par[root(x)]; } int size() const { return sz; } vector> groups() { vector> g(n); rep (i, n) if (par[i] < 0) g[i].reserve(-par[i]); rep (i, n) g[root(i)].push_back(i); vector> grp; grp.reserve(size()); rep (i, n) if (g[i].size()) grp.emplace_back(move(g[i])); return grp; } }; // <<< // >>> scc #ifndef EDGE_INFO #define EDGE_INFO constexpr int dest(int v) { return v; } template ().to)> constexpr int dest(E const& e) { return e.to; } constexpr int cost(int) { return 1; } template ().cost)> constexpr auto cost(E const& e) { return e.cost; } template using cost_t = decltype(cost(declval())); #endif template struct SCC { vector> const& g; vector ord, low, st; int32_t time = 0, pos = 0, sz = 0; UnionFind uf; SCC(vector> const& g) : g(g), ord(g.size()), low(g.size()), st(g.size()), uf(g.size()) { for (auto v : wcc()) if (ord[v] == 0) dfs(v); for (auto &x : ord) x += sz; } vector wcc() { vector nxt(g.size()), vs; vs.reserve(g.size()); rep (v, g.size()) nxt[v] = v; rep (v, g.size()) { for (auto const& e : g[v]) { auto const u = dest(e); if (uf.unite(v, u)) swap(nxt[v], nxt[u]); } } for (i32 v = (i32)g.size()-1; v >= 0; --v) { if (uf.root(v) != v) continue; auto u = v; do vs.push_back(u), u = nxt[u]; while (u != v); } return vs; } void dfs(int32_t v) { low[v] = ord[v] = ++time; st[pos++] = v; for (auto const& e : g[v]) { if (ord[dest(e)] == 0) { dfs(dest(e)); chmin(low[v], low[dest(e)]); } else if (ord[dest(e)] > 0) { chmin(low[v], ord[dest(e)]); } } if (low[v] == ord[v]) { sz++; do ord[st[--pos]] = -sz; while (st[pos] != v); } } // t-sorted id int id(int v) const { return ord[v]; } int size() const { return sz; } vector> groups() const { vector> grp(sz); rep (v, g.size()) grp[ord[v]].push_back(v); return grp; } // no multiple edges, no self-loops // no edge information vector> graph() const { vector> ret(size()); vector last(g.size(), -1); rep (v, g.size()) { for (auto const& e : g[v]) { const int32_t x = id(v), y = id(dest(e)); if (x == y or last[y] == x) continue; last[y] = x; ret[x].push_back(y); } } return ret; } }; template auto get_scc(vector> const& g) { return SCC(g); } // <<< // >>> bfs #ifndef EDGE_INFO #define EDGE_INFO constexpr int dest(int v) { return v; } template ().to)> constexpr int dest(E const& e) { return e.to; } constexpr int cost(int) { return 1; } template ().cost)> constexpr auto cost(E const& e) { return e.cost; } template using cost_t = decltype(cost(declval())); #endif template vector bfs(vector> const& g, int s, int inf) { vector d(g.size(), inf); queue q; d[s] = 0; q.push(s); while (q.size()) { int x = q.front(); q.pop(); for (auto const& e : g[x]) { const int y = dest(e); if (chmin(d[y], d[x]+1)) q.push(y); } } return d; } // <<< int32_t main() { int n, m; cin >> n >> m; vector> g(n); rep (i, m) { int x, y; cin >> x >> y; --x, --y; g[x].eb(y); } auto scc = get_scc(g); auto grp = scc.groups(); dump(grp); vector> es; for (auto const& v : grp) { if (v.size() == 1) continue; rep (i, v.size()) { int j = (i+1) % v.size(); es.eb(v[i], v[j]); } } rep (i, grp.size()-1) { int x = grp[i][0]; int y = grp[i+1][0]; if (scc.uf.same(x, y)) { es.eb(x, y); } } cout << es.size() << '\n'; for (auto [x, y] : es) cout << x+1 << " " << y+1 << '\n'; debug { vector> h(n); for (auto [x, y] : es) h[x].eb(y); rep (x, n) { for (int y : g[x]) { auto d = bfs(h, x, INF); assert(d[y] < INF); } } } }