// Slower C++ solution:
#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;

}




// Faster C++ solution for the same problem (measured speedup: 2.598849622772038x):
#include<bits/stdc++.h>

#include <ext/pb_ds/tree_policy.hpp>

#include <ext/pb_ds/assoc_container.hpp>



using namespace __gnu_pbds;

using namespace std;





template <typename T>using ordered_set = tree<T, null_type, less<T>,rb_tree_tag, tree_order_statistics_node_update>;



#define PI              acos(-1.0)

#define en '\n'

#define Infi 0x3f3f3f3f

#define LInfi 0x3f3f3f3f3f3f3f3f



#define setzero(a)    memset(a,0,sizeof(a))

#define setneg(a)     memset(a,-1,sizeof(a))

#define setinf(a)     memset(a,126,sizeof(a))





#define READ_IN freopen("Input.txt","r",stdin);

#define READ_OUT freopen("Output.txt","w",stdout);





const int dx[] = {+1,-1,+0,+0};                         // 4 X directions

const int dy[] = {+0,+0,+1,-1};                         // 4 Y directions

const int dx8[] = {+0,+0,+1,-1,-1,+1,-1,+1};            // King X Moves

const int dy8[] = {-1,+1,+0,+0,+1,+1,-1,-1};            // King Y Moves

const int kx[] = {-2, -2, -1, -1,  1,  1,  2,  2};      // Knight X Moves

const int ky[] = {-1,  1, -2,  2, -2,  2, -1,  1};      // Knight Y Moves







int  Set    (int N,int pos) {return N=N | (1<<pos);}    /// turn a bit on

int  reset  (int N,int pos) {return N= N & ~(1<<pos);}  /// turn a bit off

bool check  (int N,int pos) {return (bool)(N & (1<<pos));} /// check, if a bit is on or not



void __print(int x) {cerr << x;}

void __print(long x) {cerr << x;}

void __print(long long x) {cerr << x;}

void __print(unsigned x) {cerr << x;}

void __print(unsigned long x) {cerr << x;}

void __print(unsigned long long x) {cerr << x;}

void __print(float x) {cerr << x;}

void __print(double x) {cerr << x;}

void __print(long double x) {cerr << x;}

void __print(char x) {cerr << '\'' << x << '\'';}

void __print(const char *x) {cerr << '\"' << x << '\"';}

void __print(const string &x) {cerr << '\"' << x << '\"';}

void __print(bool x) {cerr << (x ? "true" : "false");}



template<typename T, typename V>

void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}

template<typename T>

void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}

void _print() {cerr << "]\n";}

template <typename T, typename... V>

void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}

#ifndef ONLINE_JUDGE

#define trace(x...) cerr << "[" << #x << "] = ["; _print(x)

#else

#define trace(x...)

#endif



typedef long long int  ll;

typedef unsigned long long int llu;



vector<vector<int>> graph;



int dp[100005];



int make(int id){



    if(dp[id]!=-1)return dp[id];



    int ans = 0;

    for(int i=0;i<graph[id].size();i++){

        ans = max(ans,1+make(graph[id][i]));

    }

    return dp[id] = ans;

}



int main(void){



    int n,m;

    cin >> n >> m;



    graph.assign(n+2,vector<int>());



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

        int a,b;

        cin >> a >> b;

        graph[a].push_back(b);

    }

    int ans = 0;



    memset(dp,-1,sizeof(dp));



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

        ans = max(ans,make(i));

    }



    cout << ans << endl;



    return 0;

}