#include <iostream>
#include <fstream>
#include <iomanip>
#include <climits>
#include <limits>
#include <algorithm>
#include <array>
#include <vector>
#include <deque>
#include <queue>
#include <list>
#include <stack>
#include <string>
#include <functional>
#include <numeric>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <unordered_map>
#include <random>
#include <cmath>
#include <complex>
// #include "utiltime.hpp"

using namespace std;

typedef long long int ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef pair<int, int> P;
typedef pair<ll, ll> Pll;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef complex<double> cdouble;

const int INFL = (int)1e9;
const ll INFLL = (ll)1e18;
const double INFD = numeric_limits<double>::infinity();
const double PI = 3.14159265358979323846;
#define Loop(i, n) for(int i = 0; i < (int)(n); i++)
#define Loopll(i, n) for(ll i = 0; i < (ll)(n); i++)
#define Loop1(i, n) for(int i = 1; i <= (int)(n); i++)
#define Loopll1(i, n) for(ll i = 1; i <= (ll)(n); i++)
#define Loopr(i, n) for(int i = (int)(n) - 1; i >= 0; i--)
#define Looprll(i, n) for(ll i = (ll)(n) - 1; i >= 0; i--)
#define Loopr1(i, n) for(int i = (int)(n); i >= 1; i--)
#define Looprll1(i, n) for(ll i = (ll)(n); i >= 1; i--)
#define Foreach(buf, container) for(auto buf : container)
#define Loopdiag(i, j, h, w, sum) for(int i = ((sum) >= (h) ? (h) - 1 : (sum)), j = (sum) - i; i >= 0 && j < (w); i--, j++)
#define Loopdiagr(i, j, h, w, sum) for(int j = ((sum) >= (w) ? (w) - 1 : (sum)), i = (sum) - j; j >= 0 && i < (h); j--, i++)
#define Loopdiagsym(i, j, h, w, gap) for (int i = ((gap) >= 0 ? (gap) : 0), j = i - (gap); i < (h) && j < (w); i++, j++)
#define Loopdiagsymr(i, j, h, w, gap) for (int i = ((gap) > (h) - (w) - 1 ? (h) - 1 : (w) - 1 + (gap)), j = i - (gap); i >= 0 && j >= 0; i--, j--)
#define Loopitr(itr, container) for(auto itr = container.begin(); itr != container.end(); itr++)
#define printv(vector) Loop(extra_i, vector.size()) { cout << vector[extra_i] << " "; } cout << endl;
#define printmx(matrix) Loop(extra_i, matrix.size()) { Loop(extra_j, matrix[extra_i].size()) { cout << matrix[extra_i][extra_j] << " "; } cout << endl; }
#define quickio() ios::sync_with_stdio(false); cin.tie(0);
#define readfile(filename) ifstream in(filename); cin.rdbuf(in.rdbuf());
#define bitmanip(m,val) static_cast<bitset<(int)m>>(val)
bool nearlyeq(double x, double y) { return abs(x - y) < 1e-9; }
bool inrange(int x, int t) { return x >= 0 && x < t; }
ll rndf(double x) { return (ll)(x + (x >= 0 ? 0.5 : -0.5)); }
ll floorsqrt(double x) { ll m = (ll)sqrt(x); return m + (m * m <= (ll)(x) ? 0 : -1); }
ll ceilsqrt(double x) { ll m = (ll)sqrt(x); return m + ((ll)x <= m * m ? 0 : 1); }
ll rnddiv(ll a, ll b) { return (a / b + (a % b * 2 >= b ? 1 : 0)); }
ll ceildiv(ll a, ll b) { return (a / b + (a % b == 0 ? 0 : 1)); }
ll gcd(ll m, ll n) { if (n == 0) return m; else return gcd(n, m % n); }

/*******************************************************/

typedef ll val_t;

struct graph_t {
	int n;           // |V|, index begins with 0
	int m;           // |E|
	vector<P> edges; // E
	vector<val_t> vals; // V
	vector<ll> costs; // cost or distance
	vector<ll> caps;  // capacity
};

class Topological_Sort {
private:
	struct node {
		int id; bool done; vi to; int from_cnt;
	};
	vector<node> nodes;
	int n, m;
	vi result;
	bool cycle_existing_flag;
public:
	Topological_Sort(graph_t G) {
		n = G.n;
		m = G.edges.size();
		nodes.resize(n);
		Loop(i, n) nodes[i] = { i, false,{}, 0 };
		Loop(i, m) {
			nodes[G.edges[i].first].to.push_back(G.edges[i].second);
			nodes[G.edges[i].second].from_cnt++;
		}
		result = {};
		set<int> st;
		Loop(i, n) {
			if (nodes[i].from_cnt == 0) st.insert(i);
		}
		while (st.size()) {
			int a = *(st.begin()); st.erase(a);
			result.push_back(a);
			Loop(i, nodes[a].to.size()) {
				int b = nodes[a].to[i];
				nodes[b].from_cnt--;
				if (nodes[b].from_cnt == 0) st.insert(b);
			}
		}
		if (result.size() != n) cycle_existing_flag = true;
		else cycle_existing_flag = false;
		return;
	}
	vi get_result() {
		return result;
	}
	bool is_cycle_existing() {
		return cycle_existing_flag;
	}
};

int main() {
	graph_t G; cin >> G.n;
	cin >> G.m;
	vvi backlst(G.n);
	Loop(i, G.m) {
		int s, t; cin >> s >> t; s--; t--;
		G.edges.push_back({ s, t });
		backlst[t].push_back(s);
	}
	Topological_Sort ts(G);
	vi ids = ts.get_result();
	G.vals = vll(G.n, 0);
	Foreach(a, ids) {
		Foreach(b, backlst[a]) {
			G.vals[a] = max(G.vals[a], G.vals[b] + 1);
		}
	}
	cout << *max_element(G.vals.begin(), G.vals.end()) << endl;
}

Measured memory_bytes: 15220