// Slower C++ solution:


/**

    And in the end, the love you take is equal to the love you make.



        I LOVE YOU LAURA.

**/



#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

#include <ext/rope>



#define _CRT_SECURE_NO_DEPRECATE

#define _CRT_DISABLE_PERFCRIT_LOCKS



#define findOrder find_by_order

#define orderKey order_of_key



#define isOn(mask, i) (mask & (1 << i))

#define turn(mask, i) (mask |= (1 << i))

#define switch(mask, i) (mask &= ~(1 << i))

#define change(mask, i) (mask ^= (1 << i))

#define lowbit(mask) (mask & (-mask))

#define turnAll(mask, n) (mask = (1 << n) - 1)

#define switchLastBit(mask) (mask & (mask - 1))

#define turnLastZero(mask) (mask | (mask + 1))

#define modulo(x, mod) ((x) & (mod - 1))

#define log2(x) (31 - __builtin_clz(x))



#define mkp make_pair

#define pb push_back

#define pf push_front

#define eb emplace_back

#define ef emplace_front

#define ins insert

#define rsz resize

#define f first

#define s second

#define lb lower_bound

#define ub upper_bound



#define sz(x) ((int)x.size())

#define all(x) begin(x), end(x)

#define rall(x) rbegin(x), rend(x)



#define debug(x) (cerr << x << " ")



using namespace std;

using namespace __gnu_pbds;

using namespace __gnu_cxx;



typedef double db;

typedef long long ll;

typedef long double ld;

typedef unsigned long long ull;



typedef pair<int, int> ii;

typedef pair<ll,ll> pll;

typedef pair<ld,ld> pdb;

typedef pair<int, ii> tri;

typedef complex<ld> cd;



typedef vector<int> vi;

typedef vector<ii> vii;

typedef vector<ld> vdb;

typedef vector<cd> vcd;

typedef vector<ll> vll;

typedef vector<string> vs;

typedef vector<vll> matrix;



typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_tree;



const ll mod = 1e9+7;

const int maxn = 1e5+2;

const int pi = 2 * acos(0.0);

const db eps = 1e-9;



ll n, m;

vll graph[maxn];

ll dp[maxn];



ll dfs (int node){

    ll &ans = dp[node];



    if (ans != -1) return ans;



    for (auto v: graph[node]){

        ans = max(ans, dfs(v) + 1);

    }

    return ans;

}



int main()

{

    ios_base::sync_with_stdio(0);

    cin.tie(0); cout.tie(0);



    memset(dp, -1, sizeof dp);



    cin >> n >> m;



    for (int i = 1; i <= m; i++){

        int a, b; cin >> a >> b;



        graph[a].pb(b);

    }



    ll ans = 0;

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

        ans = max(ans, dfs(i) + 1);

    }



    cout << ans << "\n";



    return 0;

}


// Faster C++ solution for the same problem (measured speedup: 4.004383500211197x):
#include <bits/stdc++.h>

typedef long long ll;

#define endl '\n'

#define pq priority_queue <pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>> 

#define graph1 vector<vector<pair<ll,ll>>>

#define graph vector<vector<ll>>

#define pq1 priority_queue<pair<ll, pair<ll, ll>>, vector<pair<ll, pair<ll,ll>>>, greater<pair<ll, pair<ll, ll>>>>

#define loop(i,a,b) for (ll i = a; i < b; i++)

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

using namespace std;

bool visited[100005];

graph g;

ll maxx = 0;

void dfs(int node,  int dp[], bool vis[])

{

	

	vis[node] = true;



	for (int i = 0; i < g[node].size(); i++) {



		if (!vis[g[node][i]])

			dfs(g[node][i], dp, vis);



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

	}

}





int findLongestPath(int n)

{

	

	int dp[n + 1];

	memset(dp, 0, sizeof dp);



	

	bool vis[n + 1];

	memset(vis, false, sizeof vis);



	

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

		if (!vis[i])

			dfs(i, dp, vis);

	}



	int ans = 0;



	// Traverse and find the maximum of all dp[i] 

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

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

	}

	return ans;

}



int main()

{



	ll n, m;

	cin >> n >> m;

	g.resize(n + 1);

	memset(visited, false, sizeof(visited));

	loop(i, 0, m)

	{

		ll u, v;

		cin >> u >> v;

		g[u].push_back(v);

	}

	cout << findLongestPath(n);

}