#include <bits/stdc++.h>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
using namespace std;
#ifdef LOCAL_DEV
	void debug_impl() { std::cerr << std::endl; }
	template<typename Head, typename... Tail> void debug_impl(Head head, Tail... tail) { std::cerr << " " << head << (sizeof...(tail) ? "," : ""); debug_impl(tail...); }
	#define debug(...) std::cerr << std::boolalpha << "(" << #__VA_ARGS__ << ") ="; debug_impl(__VA_ARGS__); std::cerr << std::noboolalpha;
#else
	#define debug(...)
#endif
#ifdef LOCAL_TEST
	#define BOOST_STACKTRACE_USE_ADDR2LINE
	#define BOOST_STACKTRACE_ADDR2LINE_LOCATION /usr/local/opt/binutils/bin/addr2line
	#define _GNU_SOURCE
	#include <boost/stacktrace.hpp>
	template<typename T> class dvector : public std::vector<T> {
	public:
		dvector() : std::vector<T>() {}
		explicit dvector(size_t n, const T& value = T()) : std::vector<T>(n, value) {}
		dvector(const std::vector<T>& v) : std::vector<T>(v) {}
		dvector(const std::initializer_list<T> il) : std::vector<T>(il) {}
		dvector(const typename std::vector<T>::iterator first, const typename std::vector<T>::iterator last) : std::vector<T>(first, last) {}
		dvector(const std::string::iterator first, const std::string::iterator last) : std::vector<T>(first, last) {}
		T& operator[](size_t n) {
			try { return this->at(n); } catch (const std::exception& e) {
				std::cerr << boost::stacktrace::stacktrace() << std::endl; return this->at(n);
			}
		}
		const T& operator[](size_t n) const {
			try { return this->at(n); } catch (const std::exception& e) {
				std::cerr << boost::stacktrace::stacktrace() << std::endl; return this->at(n);
			}
		}
	};
	class dbool {
	private:
		bool boolvalue;
	public:
		dbool() : boolvalue(false) {}
		dbool(bool b) : boolvalue(b) {}
		dbool(const dbool &b) : boolvalue(b.boolvalue) {}
		operator bool&() { return boolvalue; }
		operator const bool&() const { return boolvalue; }
	};
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::pair<T1, T2>& p) { return s << "(" << p.first << ", " << p.second << ")"; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const dvector<T>& v) {
		for (int i = 0, len = v.size(); i < len; ++i){ s << v[i]; if (i < len - 1) s << "\t"; } return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const dvector< dvector<T> >& vv) {
		for (int i = 0, len = vv.size(); i < len; ++i){ s << vv[i] << std::endl; } return s; }
	template<typename T1, typename T2> std::ostream& operator<<(std::ostream& s, const std::map<T1, T2>& m) {
		s << "{" << std::endl; for (auto itr = m.begin(); itr != m.end(); ++itr){ s << "\t" << (*itr).first << " : " << (*itr).second << std::endl; } s << "}" << std::endl; return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::set<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}" << std::endl; return s; }
	template<typename T> std::ostream& operator<<(std::ostream& s, const std::multiset<T>& se) {
		s << "{ "; for (auto itr = se.begin(); itr != se.end(); ++itr){ s << (*itr) << "\t"; } s << "}" << std::endl; return s; }
	#define vector dvector
	#define bool dbool
#endif
//#define int long long
using ll = long long;
using bll = boost::multiprecision::cpp_int;
using bdouble = boost::multiprecision::cpp_dec_float_100;
//constexpr int INF = 1e9;//INT_MAX=(1<<31)-1=2147483647
constexpr ll INF = (ll)1e18;//(1LL<<63)-1=9223372036854775807
constexpr ll MOD = (ll)1e9 + 7;
constexpr double EPS = 1e-9;
constexpr int dx[4]={1,0,-1,0};
constexpr int dy[4]={0,1,0,-1};
#define p(var) std::cout<<var<<std::endl
#define PI (acos(-1))
#define rep(i, n)   for(ll i=0, i##_length=(n); i< i##_length; ++i)
#define repeq(i, n) for(ll i=1, i##_length=(n); i<=i##_length; ++i)
#define all(v) (v).begin(), (v).end()
#define uniq(v) (v).erase(unique((v).begin(), (v).end()), (v).end());
template<typename T> inline void pv(vector<T> &v) { for(ll i=0, N=v.size(); i<N; i++) std::cout << v[i] << " \n"[i==N-1]; }
template<typename T> inline T gcd(T a, T b) { return b ? gcd(b,a%b) : a; }
template<typename T> inline T lcm(T a, T b) { return a / gcd(a,  b) * b; }
template<typename T1, typename T2> inline T1 power(T1 x, T2 n){ return n ? power(x*x%MOD,n/2)*(n%2?x:1)%MOD : 1; }
template<typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template<typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

/*-----8<-----8<-----*/

vector<ll> topological_sort(vector<vector<ll>> &g){
	ll gsize=g.size();
	//各頂点の入次数を記録
	vector<ll> indegree(gsize, 0);
	for(ll from=0; from<gsize; from++){
		for(ll to : g[from]){
			indegree[to]++;
		}
	}

	//入次数が0の頂点であればqueueに追加
	queue<ll> que;
	for(ll i=0; i<gsize; i++){
		if(indegree[i]==0)que.push(i);
	}

	//入次数が0の頂点から順にansへ格納
	vector<ll> ans;
	while(!que.empty()){
		ll v=que.front();
		que.pop();
		ans.push_back(v);
		for(ll nextv : g[v]){
			indegree[nextv]--;
			if(indegree[nextv]==0)que.push(nextv);
		}
	}

	//ansへ格納されていない頂点がある＝閉路が存在する
	return (ll)ans.size()==gsize ? ans : vector<ll>();
}

//トポロジカルソートの最大パスを返す
vector<ll> longest_path(vector<vector<ll>> &g, vector<ll> &topological){
	ll gsize=g.size();
	vector<ll> dp(gsize, 0);
	vector<ll> nextv(gsize, -1);
	ll maxval = -1;
	ll startv = -1;
	for(auto it = topological.rbegin(); it != topological.rend(); ++it){
		for(ll u : g[*it]){
			if(chmax(dp[*it], dp[u] + 1)) nextv[*it] = u;
		}
		if(chmax(maxval, dp[*it])) startv = *it;
	}

	ll v=startv;
	vector<ll> longest_path;
	while(1){
		longest_path.push_back(v);
		if(nextv[v]==-1)break;
		v=nextv[v];
	}
	return longest_path;
}


signed main() {
	ll N,M;
	cin>>N>>M;
	vector<vector<ll>> g(N);
	rep(i,M){
		ll a,b;
		cin>>a>>b;
		a--;b--;
		g[a].push_back(b);
	}

	vector<ll> t=topological_sort(g);
	vector<ll> p;
	p=longest_path(g,t);
	p(p.size()-1);
	
	return 0;
}


Measured memory_bytes: 9812