#include <bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(c) (c).begin(),(c).end()
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define MINF(a) memset(a,0x3f,sizeof(a))
#define POW(n) (1LL<<(n))
#define IN(i,a,b) (a <= i && i <= b)
using namespace std;
template <typename T> inline bool CHMIN(T& a,T b) { if(a>b) { a=b; return 1; } return 0; }
template <typename T> inline bool CHMAX(T& a,T b) { if(a<b) { a=b; return 1; } return 0; }
template <typename T> inline void SORT(T& a) { sort(ALL(a)); }
template <typename T> inline void REV(T& a) { reverse(ALL(a)); }
template <typename T> inline void UNI(T& a) { sort(ALL(a)); a.erase(unique(ALL(a)),a.end()); }
template <typename T> inline T LB(vector<T>& v, T a) { return *lower_bound(ALL(v),a); }
template <typename T> inline int LBP(vector<T>& v, T a) { return lower_bound(ALL(v),a) - v.begin(); }
template <typename T> inline T UB(vector<T>& v, T a) { return *upper_bound(ALL(v),a); }
template <typename T> inline int UBP(vector<T>& v, T a) { return upper_bound(ALL(v),a) - v.begin(); }
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) { REP(i,v.size()) { if (i) os << " "; os << v[i]; } return os; }
template <typename T> istream& operator>> (istream& is, vector<T>& v) { for(T& in : v) is >> in; return is; }
template <typename T = int> 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); }
const int MOD = 1000000007;
const int INF = 0x3f3f3f3f3f3f3f3f;
const double EPS = 1e-10;

template <typename T>
struct edge {
  int s, t;
  T c;
  edge() {}
  edge(int t, T c) : s(-1), t(t), c(c) {}
  edge(int s, int t, T c) : s(s), t(t), c(c) {}
};

template <typename T> using Edges = vector<edge<T>>;
template <typename T> using WeightedGraph = vector<Edges<T>>;
using UnWeightedGraph = vector<vector<int>>;
template <typename T> using Matrix = vector<vector<T>>;

vector<int> topological_sort(UnWeightedGraph g) {
  int n = g.size();
  vector<int> in(n);
  for (int u = 0; u < n; u++) for (int v : g[u]) in[v]++;
  stack<int> st;
  for (int u = 0; u < n; u++) if (!in[u]) st.push(u);
  vector<int> res;
  while (!st.empty()) {
    int u = st.top(); st.pop();
    res.push_back(u);
    for (int v : g[u]) if (!--in[v]) st.push(v);
  }
  return res.size() == n ? res : vector<int>();
}

signed main() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(10);

  int N,M;
  cin >> N >> M;
  UnWeightedGraph g(N);
  REP(i,M) {
    int x,y;
    cin >> x >> y;
    x--; y--;
    g[x].push_back(y);
  }
  auto v = topological_sort(g);
  vector<int> dp(N,0);
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < g[v[i]].size(); j++) {
      CHMAX(dp[g[v[i]][j]],dp[v[i]]+1);
    }
  }
  cout << *max_element(ALL(dp)) << endl;

  return 0;
}

Measured memory_bytes: 13688