/** * author: tomo0608 * created: 27.08.2021 23:35:26 **/ #ifdef __LOCAL #define _GLIBCXX_DEBUG #endif #pragma GCC optimize("Ofast") #include using namespace std; #if __has_include() #include using namespace atcoder; #endif typedef long long ll; typedef long double ld; template using V = vector; template using VV = V>; template using VVV = V>; typedef pair pii; typedef pair pll; #define all(x) x.begin(),x.end() #define SUM(v) accumulate(all(v), 0LL) #define MIN(v) *min_element(all(v)) #define MAX(v) *max_element(all(v)) #define lb(c, x) distance((c).begin(), lower_bound(all(c), (x))) #define ub(c, x) distance((c).begin(), upper_bound(all(c), (x))) #define UNIQUE(x) sort(all(x)), x.erase(unique(all(x)), x.end()) template void zip(vector &x) {vector y = x;UNIQUE(y);for(int i = 0; i < x.size(); ++i) { x[i] = lb(y, x[i]); }} #define rep1(a) for(long long i = 0; i < a; i++) #define rep2(i, a) for(long long i = 0; i < a; i++) #define rep3(i, a, b) for(long long i = a; i < b; i++) #define rep4(i, a, b, c) for(long long i = a; i < b; i += c) #define drep1(a) for(long long i = a-1; i >= 0; i--) #define drep2(i, a) for(long long i = a-1; i >= 0; i--) #define drep3(i, a, b) for(long long i = a-1; i >= b; i--) #define drep4(i, a, b, c) for(long long i = a-1; i >= b; i -= c) #define overload4(a, b, c, d, e, ...) e #define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__) #define drep(...) overload4(__VA_ARGS__, drep4, drep3, drep2, drep1)(__VA_ARGS__) templatevoid input(T&... a){(cin >> ... >> a);}; #define INT(...) int __VA_ARGS__; input(__VA_ARGS__) #define LL(...) ll __VA_ARGS__; input(__VA_ARGS__) #define STR(...) string __VA_ARGS__; input(__VA_ARGS__) #define DBL(...) double __VA_ARGS__; input(__VA_ARGS__) void print(){cout << '\n';} templatevoid print(const T& a, const Ts&... b){cout << a; (cout << ... << (cout << ' ', b)); cout << '\n';} void drop(){cout << '\n';exit(0);} templatevoid drop(const T& a, const Ts&... b){cout << a; (cout << ... << (cout << ' ', b)); cout << '\n';exit(0);} template using priority_queue_rev = priority_queue, greater >; template inline bool chmax(T &a, const U &b) { if (a inline bool chmin(T &a, const U &b) { if (a>b) { a=b; return 1; } return 0; } template istream &operator>>(istream &is, pair &p) { is >> p.first >> p.second; return is; } template ostream &operator<<(ostream &os, const pair &p) { os << '(' << p.first << ", " << p.second << ')'; return os; } template istream &operator>>(istream &is, vector &v) { for (auto &e : v) is >> e; return is; } template ostream &operator<<(ostream &os, const vector &v) { for (auto &e : v) os << e << ' '; return os; } template ostream& operator << (ostream& os, set& set_var) {os << "{"; for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {os << *itr;++itr;if(itr != set_var.end()) os << ", ";itr--;}os << "}";return os;} template ostream &operator<<(ostream &os, map &map_var) {os << "{";for(auto itr = map_var.begin(); itr != map_var.end(); itr++) {os << *itr;itr++;if (itr != map_var.end()) os << ", ";itr--;}os << "}";return os;} #ifdef __LOCAL void debug_out(){ cerr << endl;} template < class Head, class... Tail> void debug_out(Head H, Tail... T) { cerr << ' ' << H; debug_out(T...);} #define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__) #define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl #else #define debug(...) (void(0)) #define dump(x) (void(0)) #endif template inline int count_between(vector &a, T l, T r) { return lower_bound(all(a), r) - lower_bound(all(a), l); } // [l, r) #define mp make_pair #define bit(n, k) ((n >> k) & 1) /*nのk bit目*/ int topbit(signed t) { return t == 0 ? -1 : 31 - __builtin_clz(t); } int topbit(ll t) { return t == 0 ? -1 : 63 - __builtin_clzll(t); } int lowbit(signed a) { return a == 0 ? 32 : __builtin_ctz(a); } int lowbit(ll a) { return a == 0 ? 64 : __builtin_ctzll(a); } template T mypow(T x, ll n){T ret = 1;while(n > 0){if(n & 1)(ret *= x);(x *= x);n >>= 1;}return ret;} #define endl '\n' int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1}; int dy[8] = {0, 1, 0, -1, 1, -1, -1, 1}; bool visit(vector > &graph, vector & dag, vector & use, vector & finished, int s){ use[s] = false; finished[s] = false; bool c = true; for(int to: graph[s]){ if(!finished[to])return false; if(use[to])c = c && visit(graph, dag, use, finished, to); } finished[s] = true; dag.push_back(s); return c; } bool topological_sort(vector > &graph, vector & dag){// graph をもとにトポロジカルソート後の数をdagに, 閉路がある場合はfalse int n = graph.size(); bool c = true; vector use(n, true), finished(n, true); for(int i = 0; i < n; i++){ if(use[i])c = c && visit(graph, dag, use, finished, i); } reverse(all(dag)); return c; } void solve(){ INT(n,k); VV g(n); bool check = false; rep(i,k){ INT(r,c); r--,c--; g[r].emplace_back(c); } V dag; if(!topological_sort(g, dag)){ drop(-1); } reverse(all(dag)); V dist(n, -1); auto dfs = [&](int now)->ll{ if(dist[now] >= 0)return dist[now]; dist[now] = 1; for(int to: g[now])chmax(dist[now], dist[to]+1); return dist[now]; }; ll m = 0; for(int i: dag)chmax(m, dfs(i)); debug(dist); print(m); } int main() { cin.tie(0); ios_base::sync_with_stdio(false); cout << setprecision(20); int codeforces = 1; //cin >> codeforces; while(codeforces--){ solve(); } return 0; }