// Slower C++ solution:
#include <bits/stdc++.h>

/**

*   author: j o s e m a g o

*   date:

**/

using namespace std;

#pragma GCC optimize ("Ofast")

#define f first

#define s second

#define mp make_pair

#define lb lower_bound

#define ub upper_bound

#define all(v) (v).begin(),(v).end()

#define mset(a,v) memset(a,v,sizeof(a))

#define pb push_back

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

#define rsz resize

#define ln "\n"



typedef double db;

typedef unsigned long long ull;

typedef long long ll;

typedef long double ld;

const ld PI = 2*acos(0.0);

const int MOD = 1000000007;

const int JJJ = 100009;



int N,A;

vector<int>grafo[JJJ];

bool visited[JJJ];

int dp[JJJ];



int sol = 0;

void dfs(int nod){

    visited[nod] = true;

    dp[nod] = -1;

    for(int i = 0;i < (int)grafo[nod].size();i++){

        int xx = grafo[nod][i];

        if(!visited[xx]){

            dfs(xx);

        }

        dp[nod] = max(dp[nod],dp[xx]);

    }

    dp[nod]++;

}







int main(){



  

    ios_base::sync_with_stdio(0); cin.tie(0);





    cin >> N >> A;

    for(int i = 0;i < A;i++){

        int a,b;

        cin >> a >> b;

        grafo[a].pb(b);

    }



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

        if(!visited[i]){

            dfs(i);

        }

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

    }



    cout<<sol;







    return 0;

}

// Faster C++ solution for the same problem (measured speedup: 2.514676148735302x):
/*

██╗  ██╗ █████╗ ██████╗ ███████╗███████╗███╗   ███╗        ██╗  ██╗██╗  ██╗ █████╗ ██╗     ███████╗██████╗

██║ ██╔╝██╔══██╗██╔══██╗██╔════╝██╔════╝████╗ ████║        ██║ ██╔╝██║  ██║██╔══██╗██║     ██╔════╝██╔══██╗

█████╔╝ ███████║██████╔╝█████╗  █████╗  ██╔████╔██║        █████╔╝ ███████║███████║██║     █████╗  ██║  ██║

██╔═██╗ ██╔══██║██╔══██╗██╔══╝  ██╔══╝  ██║╚██╔╝██║        ██╔═██╗ ██╔══██║██╔══██║██║     ██╔══╝  ██║  ██║

██║  ██╗██║  ██║██║  ██║███████╗███████╗██║ ╚═╝ ██║███████╗██║  ██╗██║  ██║██║  ██║███████╗███████╗██████╔╝

╚═╝  ╚═╝╚═╝  ╚═╝╚═╝  ╚═╝╚══════╝╚══════╝╚═╝     ╚═╝╚══════╝╚═╝  ╚═╝╚═╝  ╚═╝╚═╝  ╚═╝╚══════╝╚══════╝╚═════╝

Believe in yourself ,( try and try and then try ), You can do it

*/

#define _CRT_SECURE_NO_WARNINGS

#include<bits/stdc++.h>

#include<unordered_map>

#define endl "\n"

#define Ceil(x,y) ((x+y-1)/y)

#define clr(arr, x) memset(arr, x, sizeof arr)

#define all(v) v.begin(),v.end()

#define allr(s) s.rbegin(),s.rend()

#define rt(s) return cout<<s,0

#define watch(x) cout<<(#x)<<" = "<<x<<endl

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

#define OO 0x3f3f3f3f3f3f3f3fLL

const int oo = 0x3f3f3f3f, mod = 1e9 + 7;

using namespace std;

typedef long long ll;

typedef long double ld;

typedef unsigned long long ull;

int dx[] { 1, -1, 0, 0, 1, 1, -1, -1 };

int dy[] { 0, 0, 1, -1, 1, -1, 1, -1 };

void K_K(){ 

	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

}

vector<vector<int>>gr;

int dp[100005];

int dfs(int node){

	if (!sz(gr[node]))

		return 0;

	int &ret = dp[node];

	if (~ret)

		return ret;

	for (auto child : gr[node]){

		ret = max(ret, dfs(child) + 1);

	}

	return ret;

}

int main(){

	K_K();

	int n, m;

	cin >> n >> m;

	gr.resize(n + 1);

	clr(dp, -1);

	for (int i = 0; i < m; i++){

		int u, v;

		cin >> u >> v;

		gr[u].push_back(v);

	}

	int ans = 0;

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

		dfs(i);

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

	}

	cout << ans;

}