#include <bits/stdc++.h>
using namespace std;
#define     all(x) x.begin(),x.end()
#define     ff first
#define     ss second
#define     ll long long
#define     ld long double
#define     pb push_back
#define     endl "\n"
#define     MOD 1000000007
#define     INF 1e18
#define     mod 998244353
#define     REP(i,n) for(ll i=0;i<(n);i++)
#define     FOR(i,a,b) for(ll i=(a);i<=(b);i++)
#define     FORD(i,a,b) for(ll i=(a);i>=(b);i--)
#define     BLOCK 555
mt19937     rng(chrono::steady_clock::now().time_since_epoch().count());

typedef     vector <ll> vll;
typedef     vector <pair<ll,ll>> vp;
typedef     vector<vector<ll> >vvll;


/*      author:- Indrajit Sen
        Institution:- Jalpaiguri Government Engineering College
        id:- @Is97, @indra1997
                                                                */
                                                                

                                                            


// log(y) power calculation
ll power_mod(ll x, ll y, ll m){
    ll arr = 1;
    x%=m;
    while(y){
        if(y&1){
            arr=(arr*x)%m;
        }
        y>>=1;
        x=(x*x)%m;
    }
    return arr;
}

ll power(ll x, ll y){
    ll arr = 1;
    while(y){
        if(y&1){
            arr*=x;
        }
        y>>=1;
        x = x*x;
    }
    return arr;
}


ll mod_inv(ll x,ll m){
    return power_mod(x,m-2,m);
}

int randomGenerator(int limit){ 
    
        random_device rd; 
        mt19937 gen(rd()); 
        uniform_int_distribution<> dis(1, limit);

    return dis(gen);
}

// customized hash function for lowering the collision in UNORDERED_MAP
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
       
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }
 
    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
 
 

bool comp(const pair<ll,pair<ll,ll>> &a, const pair<ll,pair<ll,ll>> &b) { 
    if(a.ff == b.ff){

        return (a.second < b.second); 
    }
    return (a.ff < b.ff); 
} 

struct hash_pair { 
    template <class T1, class T2> 
    size_t operator()(const pair<T1, T2>& p) const
    { 
        auto hash1 = hash<T1>{}(p.first); 
        auto hash2 = hash<T2>{}(p.second); 
        return hash1 ^ hash2; 
    } 
};

bool isPalindrome(string s){
    bool flag = true;
    for(int i=0;i<s.length()/2;i++){
        if(s[i]!=s[s.length()-i-1]){
            flag = false;
            break;
        }
    }
    return flag;
}

ll n,m;
vll adj[100005];
ll dp[100005];

ll dfs(ll node, ll len){
    if(adj[node].size()==0){
        return 0;
    }
    for(ll child:adj[node]){
        ll child_len = dp[child];
        if(child_len==-1){
            len = max(len,1 + dfs(child,0));
        }
        else{
            len = max(len,1 + child_len);
        }
    }
    return dp[node] = len;
}

void solve(){

    cin>>n>>m;
    REP(i,m){
        ll x,y;
        cin>>x>>y;
        adj[x].pb(y);
    }
    memset(dp,-1,sizeof(dp));
    ll longest_path = -1;
    FOR(i,1,n){
        if(dp[i]==-1){
            longest_path = max( dfs(i,0), longest_path);
        }
        else{
            longest_path = max(dp[i], longest_path);
        }
        // cout<<dp[i]<<" ";
    }
    /*cout<<endl;*/
    cout<<longest_path<<endl;

}


int main(){
    
    ios_base::sync_with_stdio(false); 
    cin.tie(NULL);

    solve();

    return 0;
}

Measured memory_bytes: 8576