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//#pragma optimization_level 3

//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <bitset>
#include <chrono>
#include <ctime>
#include <queue>
#include <math.h>
#include <deque>
#include <stack>
#include <iomanip>
#include <assert.h>
#include <stdio.h>
#include <cstring>
#include <random>

using namespace std;

#define int long long
#define ll long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define pld pair<ld, ld>
#define vi vector <int>
#define vpi vector <pii>
#define vld vector <ld>
#define try tr
#define left left11
#define right right11
#define lb lower_bound
#define ub upper_bound
#define sz(c) (int)(c).size()
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define pf push_front
#define sqr(a) ((a) * (a))
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);

#define x first
#define y second

mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());

clock_t START;

inline ld TIME() {
    return (ld) (clock() - START) / CLOCKS_PER_SEC;
}

ld TL = 2.0; 
int calls = 0;

inline bool IS() {
    if(++calls == 1000) {
        calls = 0;
        if(TL - TIME() < 0.1)
            return true;
    }
    return false;
}

template<typename T1, typename T2>
inline void amin(T1 &a, T2 b) { if (a > b) a = b; }

template<typename T1, typename T2>
inline void amax(T1 &a, T2 b) { if (a < b) a = b; }

template<typename T1, typename T2>
ostream &operator<<(ostream &os, pair<T1, T2> &p) {
    os << p.first << ' ' << p.second << endl;
    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>
istream &operator>>(istream &is, vector<T> &v) {
    for (auto &u : v) is >> u;
    return is;
}

template<typename T>
ostream &operator<<(ostream &os, vector<T> &v) {
    for (auto &u : v) os << u << ' ';
    os << endl;
    return os;
}

int hash_mod[4] = {1000000007, 998244353, 1000000009, 999999937}, mod = hash_mod[rnd() % 4];
int hash_pows[4] = {179, 239, 1007, 2003}, P = hash_pows[rnd() % 4];

int binpow(int x, int p) {
    int res = 1;
    while(p) {
        if(p % 2)
            res = (res * x) % mod;
        x = (x * x) % mod;
        p /= 2;
    }
    return res;
}

const int N = 1e6 + 7, NS = 3e3 + 7, lg = 20, sq = 550, inf = 2e9 + 7, SM = 1e2 + 7;
const ld eps = 1e-9;
int d[N], used[N];
vi v[N], o;
void ts(int x){
	used[x] = 1;
	for (auto u : v[x]) {
		if(!used[u])	
			ts(u);
	}
	o.pb(x);
}
int dfs(int x) {
	used[x] = 1;
	d[x] = 1;
	for (auto u : v[x]) {
		if(!used[u])
			dfs(u);
		amax(d[x], d[u] + 1);		
	}
	return d[x];
}
void solve() {
	int n, m;
	cin >> n >> m;
	for (int i = 0; i < m; i++) {
		int a, b;
		cin >> a >> b;
		a--, b--;
		v[a].pb(b);
	}
	for (int i = 0; i < n; i++) {
		if(!used[i])
			ts(i);
	}
	reverse(all(o));
	int ans = 0;
	fill(used, used + n, 0);
	for (auto u : o) {
		if(!used[u])
			amax(ans, dfs(u));
	}
	cout << ans - 1;
}

signed main() {
    fast
    solve();
}

Measured memory_bytes: 38424