#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
//#include <boost/multiprecision/cpp_int.hpp>
using namespace std;

using dou =long double;
string yes="yes";
string Yes="Yes";
string YES="YES";
string no="no";
string No="No";
string NO="NO";

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
typedef long long ll;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;
const ll mod = 1000000007ll;
//const ll mod = 10000;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint operator-() const { return mint(-x);}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod-a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) { (x *= a.x) %= mod; return *this;}
  mint operator+(const mint a) const { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= a;}
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod-2);}
  mint& operator/=(const mint a) { return *this *= a.inv();}
  mint operator/(const mint a) const { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}




#define rep(i, n)         for(ll i = 0; i < (ll)(n); i++)
#define brep(n)           for(int bit=0;bit<(1<<n);bit++)
#define bbrep(n)           for(int bbit=0;bbit<(1<<n);bbit++)
#define erep(i,container) for (auto &i : container)
#define itrep(i,container) for (auto i : container)
#define irep(i, n)        for(ll i = n-1; i >= (ll)0ll; i--)
#define rrep(i,m,n) for(ll i = m; i < (ll)(n); i++)
#define reprep(i,j,h,w) rep(i,h)rep(j,w)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define VEC(type,name,n) std::vector<type> name(n);rep(i,n)std::cin >> name[i];
#define pb push_back
#define pf push_front
#define query int qq;std::cin >> qq;rep(qqq,qq)
#define lb lower_bound
#define ub upper_bound
#define fi first
#define se second
#define itn int
#define mp make_pair
#define sum(a) accumulate(all(a),0ll)
#define keta fixed<<setprecision
#define vout(a) erep(qxqxqx,a)std::cout << qxqxqx << ' ';std::cout  << std::endl;
#define vvector(name,typ,m,n,a)vector<vector<typ> > name(m,vector<typ> (n,a))
//#define vvector(name,typ,m,n)vector<vector<typ> > name(m,vector<typ> (n))
#define vvvector(name,t,l,m,n,a) vector<vector<vector<t> > > name(l, vector<vector<t> >(m, vector<t>(n,a)));
#define vvvvector(name,t,k,l,m,n,a) vector<vector<vector<vector<t> > > > name(k,vector<vector<vector<t> > >(l, vector<vector<t> >(m, vector<t>(n,a)) ));
#define case std::cout <<"Case #" <<qqq+1<<":"
#define RES(a,i,j) a.resize(i);rep(ii,i)a[ii].resize(j);
#define RESRES(a,i,j,k) a.resize(i);rep(ii,i)a[ii].resize(j);reprep(ii,jj,i,j){dp[ii][jj].resize(k)};
#define res resize
#define as assign
#define ffor for(;;)
#define ppri(a,b) std::cout << a<<" "<<b << std::endl
#define pppri(a,b,c) std::cout << a<<" "<<b <<" "<< c<<std::endl
#define aall(x,n) (x).begin(),(x).begin()+(n)
#define ssum(a) accumulate(a,0ll) 
#define stirng string
#define gin(a,b) int a,b;std::cin >> a>>b;a--;b--;
#define popcount __builtin_popcount
#define permu(a) next_permutation(all(a))
//#define grid_input(a,type) int h,w;std::cin >> h>>w;vvector(a,type,h,w,0);reprep(i,j,h,w)std::cin >> a[i][j];

//typedef long long T;
ll ceil(ll a,ll b){
    return ((a+b-1)/b);
}
const int INF = 2000000000;
//const ll INF64 =3223372036854775807ll;
//const ll INF64 = 9223372036854775807ll;
const ll INF64 = 9223372036854775ll;
const ll MOD = 1000000007ll;
//const ll MOD = 1000003ll;
const ll OD = 1000000000000007ll;
const dou pi=3.141592653589793;
long long modpow(long long a, long long n) { //累乗の余剰
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % MOD;
        a = a * a % MOD;
        n >>= 1;
    }
    return res;
}

//メモ
//resizeはメンバ関数
//三項演算子を覚えよう


std::vector<int> c;//トポロジカルソート
//topo(g)で実行、returnされたvectorのsizeがnじゃないときはサイクルが存在するというサイクル検出に使えるはず
//(こどふぉでそんなやつを書いたときはうまく通った)
std::set<int> al;
void topological(int p,std::vector<std::vector<int>> &g,std::vector<int> &ans){
    ans.pb(p);
    al.erase(p);
    erep(i,g[p]){
         c[i]--;
         if(c[i]==0){
             topological(i,g,ans);
         }
    }
    //return;
}
std::vector<int> topo(std::vector<std::vector<int>> &g){
    c.clear();
    al.clear();
    std::vector<int> ans;
    rep(i,g.size())al.insert(i);
    c.resize(g.size());
    rep(i,g.size()){
        erep(j,g[i]){
            c[j]++;
        }
    }
    rep(i,g.size()){
        if(c[i]==0&&al.find(i)!=al.end()){
            topological(i,g,ans);
        }
    }
    //std::cout << ans.size() << std::endl;
    return ans;
}


int main(){
    int n,m;
    std::cin >> n>>m;
    std::vector<std::vector<int>> g(n);
    rep(i,m){
        gin(a,b);
        g[a].pb(b);
        //g[b].pb(a);
    }
    std::vector<int> t=topo(g);
    std::vector<int> dp(n);
//    vout(t);
    rep(i,n){
        erep(j,g[t[i]])chmax(dp[j],dp[t[i]]+1);
    }
    int ans=0;
    //vout(dp);
    rep(i,n){
        chmax(ans,dp[i]);
    }
    std::cout << ans << std::endl;


}

Measured memory_bytes: 21980