#include<bits/stdc++.h>
#ifdef ONLINE_JUDGE
#define endl '\n'
#ifndef CODE_JAM
#undef CASE_INFO
#endif
#else
#pragma GCC optimize "trapv"
#endif
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define rep(n) for(int i = 0; i < n; i++)
#define repj(n) for(int j = 0; j < n; j++)
#define all(p) p.begin(), p.end()
#define count_1(p) __builtin_popcountll(p)
#define count_0(p) __builtin_ctzll(p)
template<class X>inline X mid(X s,X e){return (s+(e-s)/2);}
template<class X>inline X len(X s,X e){return (e-s+1);}
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<int, ll> pil;
typedef pair<ll, int> pli;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
const int MOD = 1e9 + 7;
const int oo = 987654321;
const ll OO = 1e18;
template <typename A, typename B>
ostream& operator <<(ostream& out, const pair<A, B>& a) {
  out << "(" << a.first << "," << a.second << ")";
  return out;
}
template <typename T, size_t N>
ostream& operator <<(ostream& out, const array<T, N>& a) {
  out << "["; bool first = true;
  for (auto& v : a) { out << (first ? "" : ", "); out << v; first = 0;} out << "]";
  return out;
}
template <typename T>
ostream& operator <<(ostream& out, const vector<T>& a) {
  out << "["; bool first = true;
  for (auto& v : a) { out << (first ? "" : ", "); out << v; first = 0;} out << "]";
  return out;
}
template <typename T, class Cmp>
ostream& operator <<(ostream& out, const set<T, Cmp>& a) {
  out << "{"; bool first = true;
  for (auto& v : a) { out << (first ? "" : ", "); out << v; first = 0;} out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream& operator <<(ostream& out, const map<U, T, Cmp>& a) {
  out << "{"; bool first = true;
  for (auto& p : a) { out << (first ? "" : ", "); out << p.first << ":" << p.second; first = 0;} out << "}";
  return out;
}
#ifndef ONLINE_JUDGE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...) 42
#endif
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}

class Timer {
	clock_t start;
public:
	Timer() {start = clock();}
	~Timer() {cerr << (double)(clock() - start)/CLOCKS_PER_SEC << "\n";}
};

void solve() {
	int n, m;
	cin >> n >> m;
	vector<list<int>> adj(n);
	vector<int> in(n);
	for(int i = 0; i < m; i++) {
		int u, v;
		cin >> u >> v;
		adj[--u].push_back(--v);
		in[v]++;
	}
	queue<int> q;
	vector<int> top;
	for(int i = 0; i < n; i++) if(in[i] == 0) q.push(i);
	while(q.size()) {
		int u = q.front(); q.pop();
		top.push_back(u);
		for(const int& v : adj[u]) {
			if(--in[v] == 0) q.push(v);
		}
	}
	vector<int> dp(n);
	int ans = 0;
	for(const int& u : top) {
		for(const int&v : adj[u]) dp[v] = max(dp[v], 1 + dp[u]);
	}
	trace(top);
	trace(dp);
	cout << *max_element(all(dp)) << endl;
}

signed main() {
	Timer ti;
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr); cout.tie(nullptr);
	int T = 1;
	#ifdef TEST_CASES
	cin >> T;
	#endif
	for(int t = 1; t <= T; t++) {
		#ifdef CASE_INFO
		cout << "Case #" << t << ": ";
		#endif
		solve();
	}
	return 0;
}


Measured memory_bytes: 10336