// 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: 1.2032212589205489x):
#include<bits/stdc++.h>



typedef   long long  int ll;

typedef  long double ld;

typedef std::vector<ll> vi;

typedef std::vector<std::vector<ll> > vv;



#define MOD 1000000007

#define IO ios_base::sync_with_stdio(false);cin.tie(NULL);

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

#define mp make_pair

#define pb(x) push_back(x)

#define fi first

#define se second

#define print(vec,a,b) for(ll i=a;i<b;i++) cout<<vec[i]<<" ";cout<<endl;

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

#define input(vec,a,b) for(ll i = a;i<b;i++) cin>>vec[i];

#define ms(a,val) memset(a,val,sizeof(a))

using namespace std;



const int N = 1e5 + 5;

vi adj[N];

ll path = 0;

bool vis[N];

stack<ll> arr;

void top_sort(ll u)

{

    vis[u] = true;

    //cout<<"dfs at"<<u<<endl;

    

    for(int to : adj[u])

    {

		

		if(!vis[to])

			top_sort(to);

	}

	//cout<<u<<" has been pushed"<<endl;

	arr.push(u);



	

}

void solve(){

            ll i,j,n,m;

           cin>>n>>m;

           fo(i,0,m)

           {

			   ll a,b;

			   cin>>a>>b;

			   adj[a].pb(b);

		   }

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

		   {

			   if(!vis[i])

			   top_sort(i);

		   }

		   vi ord;

		   while(!arr.empty())

		   {

			   ord.pb(arr.top());

			   arr.pop();

		   }

		   //print(ord,0,ord.size());

		   ms(vis,0);

		   ll dis[n+1] ={0}; 

		   for(auto it : ord)

		   {

			   for(auto to : adj[it])

			   {

				   //cout<<it<<" "<<to<<endl;

				   dis[to] = max(dis[to],dis[it]+ 1);

			   }

			     

			}

			//print(dis,1,n+1);

			cout<<*max_element(dis+1,dis+n+1)<<endl;

		   

		   

            

						            



















}



int main()

{

    IO;

      ll t=1;

      //cin>>t;





      while(t--)

      {



          solve();

      }

    return 0;

}