/*
このコード、と～おれ!
Be accepted!
∧＿∧　
（｡･ω･｡)つ━☆・*。
⊂　　 ノ 　　　・゜+.
　しーＪ　　　°。+ *´¨)
 　　　　　　　　　.· ´¸.·*´¨) ¸.·*¨)
		  　　　　　　　　　　(¸.·´ (¸.·'* ☆
*/

#include <cstdio>
#include <algorithm>
#include <string>
#include <cmath>
#include <cstring>
#include <vector>
#include <numeric>
#include <iostream>
#include <random>
#include <map>
#include <unordered_map>
#include <queue>
#include <regex>
#include <functional>
#include <complex>
#include <list>
#include <cassert>
#include <iomanip>
#include <set>
#include <stack>
#include <bitset>

////多倍長整数, cpp_intで宣言
//#include <boost/multiprecision/cpp_int.hpp>
//using namespace boost::multiprecision;

//#pragma gcc target ("avx2")
//#pragma gcc optimization ("Ofast")
//#pragma gcc optimization ("unroll-loops")
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define printynl(a) printf(a ? "yes\n" : "no\n")
#define printyn(a) printf(a ? "Yes\n" : "No\n")
#define printYN(a) printf(a ? "YES\n" : "NO\n")
#define printim(a) printf(a ? "possible\n" : "imposible\n")
#define printdb(a) printf("%.50lf\n", a) //少数出力
#define printLdb(a) printf("%.50Lf\n", a) //少数出力
#define printdbd(a) printf("%.16lf\n", a) //少数出力(桁少なめ)
#define prints(s) printf("%s\n", s.c_str()) //string出力
#define all(x) (x).begin(), (x).end()
#define deg_to_rad(deg) (((deg)/360.0L)*2.0L*PI)
#define rad_to_deg(rad) (((rad)/2.0L/PI)*360.0L)
#define Please return
#define AC 0
#define manhattan_dist(a, b, c, d) (abs(a - c) + abs(b - d)) /*(a, b) から (c, d) のマンハッタン距離 */
#define inf numeric_limits<double>::infinity();
#define linf numeric_limits<long double>::infinity()

using ll = long long;
using ull = unsigned long long;

constexpr int INF = 1073741823;
constexpr int MINF = -1073741823;
constexpr ll LINF = ll(4661686018427387903);
constexpr ll MOD = 1e9 + 7;
constexpr long double eps = 1e-6;
const long double PI = acosl(-1.0L);

using namespace std;

void scans(string& str) {
	char c;
	str = "";
	scanf("%c", &c);
	if (c == '\n')scanf("%c", &c);
	while (c != '\n' && c != -1 && c != ' ') {
		str += c;
		scanf("%c", &c);
	}
}

void scanc(char& str) {
	char c;
	scanf("%c", &c);
	if (c == -1)return;
	while (c == '\n') {
		scanf("%c", &c);
	}
	str = c;
}


double acot(double x) {
	return PI / 2 - atan(x);
}

ll LSB(ll n) { return (n & (-n)); }

template<typename T>
T chmin(T& a, const T& b) {
	if (a > b)a = b;
	return a;
}

template<typename T>
T chmax(T& a, const T& b) {
	if (a < b)a = b;
	return a;
}

/*-----------------------------------------ここからコード-----------------------------------------*/

/*
* @title template(graph)
* @docs kyopro/docs/graph_template.md
*/

template<typename T>
struct edge {
	T cost;
	int from, to;

	edge(int from, int to) : from(from), to(to), cost(T(1)) {}
	edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}
};

template<typename T = int>
struct graph {

	int n;
	bool directed, weighted;

	vector<vector<edge<T>>> g;

	graph(int n, bool directed, bool weighted) : g(n), n(n), directed(directed), weighted(weighted) {}

	void add_edge(int from, int to, T cost = T(1)) {
		g[from].emplace_back(from, to, cost);
		if (not directed) {
			g[to].emplace_back(to, from, cost);
		}
	}

	vector<edge<T>>& operator[](const int& idx) {
		return g[idx];
	}

	void read(int e, bool one_indexed) {
		int a, b, c = 1;
		while (e--) {
			scanf("%d%d", &a, &b);
			if (weighted) {
				scanf("%d", &c);
			}
			if (one_indexed)--a, --b;
			add_edge(a, b, c);
		}
	}

	void read(int e, bool one_indexed, const string& format) {
		int a, b;
		T c = T(1);
		while (e--) {
			scanf("%d%d", &a, &b);
			if (weighted) {
				scanf(format, &c);
			}
			if (one_indexed)--a, --b;
			add_edge(a, b, c);
		}
	}

};

/*
* @title topological-sort
* @docs kyopro/docs/topological_sort.md
*/

template<typename T = int>
vector<int> topological_sort(graph<T>& g, const int& v) {
	vector<int> ret, in(v);

	queue<int> que;
	rep(i, v) for (const auto& aa : g[i])++in[aa.to];
	rep(i, v)if (not in[i])que.push(i);

	while (not que.empty()) {
		int top = que.front();
		que.pop();
		for (const auto& aa : g[top]) {
			--in[aa.to];
			if (not in[aa.to])que.push(aa.to);
		}
		ret.emplace_back(top);
	}
	return ret;
}

int main() {

	int n, m;
	scanf("%d%d", &n, &m);
	graph g(n, true, false);
	g.read(m, true);
	vector<int> dp(n);
	for(const auto &a: topological_sort(g, n)) for (const auto& aa : g[a])chmax(dp[aa.to], dp[a] + 1);
	printf("%d\n", *max_element(all(dp)));

	Please AC;
}


Measured memory_bytes: 10184