/*
                    !!!!!!                                                     
                    !!!!!!!!!!                                                 
                     !!!!!!!!!!!!!!!                                           
                       !!!!!!!!!!!!!!                                          
                     !!!!!!!!!!!!!!!                                           
                      !!!!!!!!!!!!                                             
                      !!!!!!!!!!!!                                             
                        !!!!!!!!!!!!                                           
                        !!!!!!!!                                               
                        !!!!!!!!!!                                             
                       !!!!!!!!!!!!!!                                          
                     !!!!!!!!!!!!!!!!                                          
                    !!!!!!!!!!!!!!!!                                  !!!!!    
                  !!!!!!!!!!!!!!!!!!!                               !!!!!!!!!! 
                 !!!!!!!!!!!!!!!!!!!!!!!                 !         !!!!!!!!!!  
            !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!              !!     !!!!!!!!!!!!    
           !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!        !!      !!!!!!!!       
            !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!      
             !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!       
              !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  !!!!!!!!!!!!       
       !!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!        !!!!!!        
      !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!      !!!!!         
          !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!        !!!          
        !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!        !          
          !!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!                       
           !!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!                         
                  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!                          
                 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!                           
                  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!                               
                  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!                               
                  !!!!!!!!!!!!!!!!!!!!!!!!!!!!                                 
                  !!!!!!!!!!!!!!!!!!!!!!!!!!                                   
                  !!!!!!!!!!!!!!!!!!!!!!!!!                                    
                   !!!!!!!!!!!!!!!!!!!!!!!!                                    
                    !!!!!!!!!!!!!!!!!!!!                                       
                    !!!!!!!!!!!!!!!!!!!                                        
                     !!!!!!!!!!!!!!!!                                          
                      !!!!!!!!!!!!!!!!                                         
                      !!!!!!!!!!!!!!!                                          
                       !!!!!!!!!!!!!!                                          
                        !!!!!!!!!!!!                                           
                        !!!!!!!!!!!!                                           
                        !!!!!!!!!!!!                                           
                          !!!!!!!!                                             
                          !!!!!!                                               
                           !!!! 
*/                                               
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/trie_policy.hpp>

#define MAXN 200005
#define mod 1000000007 
#define inf LLONG_MAX/3
#define pb push_back
using namespace std;
/*
 *  author  :  vivekcrux
 */
typedef long long ll; 
typedef pair<ll,ll> pll;


ll power(ll x,ll y,ll m=mod)
{
  ll ans = 1;
  while(y>0)
  {
    if(y&1)
    {
      ans = (ans*x)%m;
    }
    x = (x*x)%m;  
    y /= 2;
  }
  return ans%m;
}

ll modInverse(ll a, ll m=mod) 
{ 
    ll m0 = m; 
    ll y = 0, x = 1; 
  
    if (m == 1) 
      return 0; 
  
    while (a > 1) 
    { 
        // q is quotient 
        ll q = a / m; 
        ll t = m; 
  
        // m is remainder now, process same as 
        // Euclid's algo 
        m = a % m, a = t; 
        t = y; 
  
        // Update y and x 
        y = x - q * y; 
        x = t; 
    } 
  
    // Make x positive 
    if (x < 0) 
       x += m0; 
  
    return x; 
}

void toposort(ll n, vector<ll> g[], ll vis[], stack<ll> &st)
{
  vis[n] = true;
  for(auto it: g[n])
  {
    if(!vis[it])
    {
      toposort(it, g, vis, st);
    }
  }
  st.push(n);
}

int main()
{
  /*#ifndef ONLINE_JUDGE 
  freopen("input.txt", "r", stdin); 
  freopen("output.txt", "w", stdout); 
  #endif*/

  ios_base::sync_with_stdio(false);
  cin.tie(NULL);

  ll T=1;
  // cin>>T;  
  while(T--)
  {
    ll n,k,m,i,j,ans=0;
    cin>>n>>m;
    vector<ll> g[n+1],p[n+1];
    for(i=0;i<m;i++)
    {
      ll u,v;
      cin>>u>>v;
      g[u].push_back(v);
      p[v].push_back(u);
    }
    vector<ll> dist(n+1,0);
    ll vis[n+1];
    memset(vis, 0, sizeof(vis));
    stack<ll> st;
    for(i=1;i<=n;i++)
    {
      if(!vis[i])
        toposort(i, g, vis, st);
    }
    while(!st.empty())
    {
      ll node = st.top();
      st.pop();
      for(auto it: p[node])
      {
        dist[node] = max(dist[node], dist[it]+1);
      }
      ans = max(ans, dist[node]);
    }
    cout<<ans;
  }
  return 0;
}


Measured memory_bytes: 16384