// Slower C++ solution:
#include <iostream>

#include <stdio.h>

#include <string>

#include <cstring>

#include <cstdio>

#include <algorithm>

#include <cmath>

#include <vector>

#include <set>

#include <map>

#include <unordered_map>

#include <queue>

#include <deque>

#include <stack>

#include <bitset>

#include <iomanip>



#define ll long long

#define ld long double

#define ull unsigned long long

#define endl "\n"

#define MohamedMotaz ios::sync_with_stdio(0); cin.tie(0); ios_base::sync_with_stdio(0);

#define f(a, b, c) for(int a = b; a < c; a++)

#define fr(a, b, c) for(int a = b; a >= c; a--)

using namespace std;





//ll fact[1000001]; ll inv[1000001];

//ll primes[100007];

//ll arr[1000007];

//ll modPower(ll b, ll p) {

//	if (p == 0)

//		return 1;

//

//	ll halfpow = modPower(b, p / 2);

//	ll toReturn = (halfpow * halfpow) % mod;

//	if (p % 2)

//		toReturn = (toReturn * b) % mod;

//

//	return toReturn;

//}

//

//ll fastPower(ll b, ll p) {

//	if (p == 0)

//		return 1;

//	ll ans = fastPower(b, p / 2);

//	ans = (ans * ans);

//	if (p % 2 != 0)

//		ans = (ans * b);

//	return ans;

//}

//ll GcdRecursive(ll a, ll b) {

//	if (b == 0) return a;

//	return GcdRecursive(b, a % b);

//}

//ll modLCM(ll a, ll b) {

//	ll val = GcdRecursive(a, b);

//	ll tmp = ((a % mod) * (b % mod)) % mod;

//	ll finalVal = ((tmp % mod) * (arr[val] % mod)) % mod;

//	return finalVal;

//

//}

//ll LCM(ll a, ll b) {

//	return (a * b) / GcdRecursive(a, b);

//}

//void move1step(ll& a, ll& b, ll q) { // a and b by reference

//	ll c = a - q * b;

//	a = b;

//	b = c;

//}

//ll GcdIterative(ll a, ll b) {

//	while (b) move1step(a, b, a / b);

//	return a;

//}

//

//void pre(ll n) {

//

//	fact[0] = 1;

//	inv[0] = 1;

//

//	for (ll i = 1; i <= n; i++) {

//		fact[i] = (i * fact[i - 1]) % mod;

//		inv[i] = modPower(fact[i], mod - 2);

//		arr[i] = modPower(i, mod - 2);

//	}

//}

//

//ll npr(ll n, ll r) {

//	return ((fact[n] * inv[n - r]) % mod);

//}

//

//ll ncr(ll n, ll r) {

//	return ((((fact[n] * inv[n - r]) % mod) * inv[r]) % mod);

//}

//

//void sieve(ll val) {

//	memset(primes, 1, sizeof primes);

//	primes[0] = primes[1] = false;

//	for (int i = 2; i <= val; i++) {

//		if (primes[i]) {

//			for (int j = i * i; j <= val; j += i) {

//				primes[j] = 0;

//			}

//		}

//	}

//

//}



const ll mod = 1e9 + 7;

const ll N = 2e5 + 7;

const ll inf = 1e9 + 5;







ll n,m, a, b, ans = 0, c= 0;

vector<ll> adj[N];

bool visited[N] = {false};

ll dp[N] = {};

void dfs(ll node){





    visited[node] = true;

    for (auto elem: adj[node]){

        if (!visited[elem]) dfs(elem);

        dp[node] = max(dp[node], dp[elem] + 1);

        ans = max(ans, dp[node]);

    }





}



int main()

{

    MohamedMotaz;

    cin >> n >> m;

    f(i,0,m){

        cin >> a >> b;

        adj[a].push_back(b);

    }



    for (int i = 1; i <= n; i++) dfs(i);



    cout << ans;

}








// Faster C++ solution for the same problem (measured speedup: 1.8438178959267029x):
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

typedef unsigned long long ull;

typedef pair<int, int> pii;

const int INF = 0x3f3f3f3f;

const int maxn = 1e5 + 9;

inline int read()

{

    int data = 0, f = 1;

    char ch = getchar();

    while (!isdigit(ch))

    {

        if (ch == '-')

            f = -1;

        ch = getchar();

    }

    while (isdigit(ch))

    {

        data = (data << 3) + (data << 1) + ch - '0';

        ch = getchar();

    }

    return f * data;

}

vector<int> edge[maxn];

int dis[maxn], ans;

bool vis[maxn];

void dfs(int x)

{

    if (vis[x])

        return;

    vis[x] = true;

    for (int i = 0; i < edge[x].size(); ++i)

    {

        dfs(edge[x][i]);

        dis[x] = max(dis[x], dis[edge[x][i]] + 1);

    }

    ans = max(ans, dis[x]);

}

int main()

{

    //freopen("in.txt","r",stdin);

    //freopen("data.txt","w",stdout);

    //std::ios::sync_with_stdio(false);

    //std::cin.tie(0);

    int n = read(), m = read();

    while (m--)

    {

        int x = read(), y = read();

        edge[x].emplace_back(y);

    }

    for (int i = 1; i <= n; ++i)

        dfs(i);

    cout << ans << '\n';

    //fclose(stdin);

    //fclose(stdout);

    return 0;

}