#include <bits/stdc++.h>

// #include <iostream> // cout, endl, cin
// #include <string> // string, to_string, stoi
// #include <vector> // vector
// #include <algorithm> // min, max, swap, sort, reverse, lower_bound, upper_bound
// #include <utility> // pair, make_pair
// #include <tuple> // tuple, make_tuple
// #include <cstdint> // int64_t, int*_t
// #include <cstdio> // printf
// #include <map> // map
// #include <queue> // queue, priority_queue
// #include <set> // set
// #include <stack> // stack
// #include <deque> // deque
// #include <unordered_map> // unordered_map
// #include <unordered_set> // unordered_set
// #include <bitset> // bitset
// #include <climits>
// #include <cmath>
// #include <iomanip>
// #include <functional>
// #include <numeric>
// #include <random>

using namespace std;
    
#define int long long
#define pb push_back
#define F first
#define S second
#define FOR(i,a,b) for(int (i)=(a);(i)<(b);(i)++)
#define rep(i,n) FOR(i,0,n)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
#define ve vector
#define vi vector<int>
#define vp vector<pair<int,int>>
#define vvi vector<vector<int>>

template<typename T> using pq = priority_queue<T,vector<T>,greater<T>>; 
using ll = long long;
ll INF = LLONG_MAX / 4000 - 100;
ll mod = 1e9 + 7;
int dx[] = {-1,0,1,0}, dy[] = {0,1,0,-1};
vector<ll> prime;
    
class fact {
public:
    int fmod = 1e9+7;
    vector<int> fac, finv, inv;
    fact (int n, int Mod = 1e9+7) {
        fmod = Mod;
        fac = vector<int>(n,0);
        finv = vector<int>(n,0);
        inv = vector<int>(n, 0);
        fac[0] = 1; for (int i = 1; i < n; i++) fac[i] = fac[i-1] * i % fmod;
        for (int i = 0;i < n;i++) finv[i] = fact::POW(fac[i], fmod-2);
        for (int i = 0;i < n;i++) inv[i] = POW(i, fmod-2);
    }
    ll nCr(ll n, ll r) {if(n < r) return 0; return (fac[n] * finv[r] % fmod) * finv[n-r] % fmod;}
    ll POW(ll a, ll b) {ll c = 1; while (b > 0) {if (b & 1) {c = a * c%fmod;}a = a * a%fmod; b >>= 1;}return c;}
    inline int operator [] (int i) {return fac[i];}
};
    
template <class T = ll> T in() {T x; cin >> x; return (x);}
void DEBUG(vector<int> a) {for(int i=0;i<a.size();i++)cout<<a[i]<<" ";cout<<endl;}
void EMP(int x) {cout<<"!!!"<<x<<"!!!"<<endl;}
ll GCD(ll a, ll b) {ll c; while (b != 0) {c = a % b; a = b; b = c;}return a;}
ll LCM(ll a, ll b) {return (a / GCD(a, b)) * (b / GCD(a, b)) * GCD(a, b);}
ll POW(ll a, ll b) {ll c = 1; while (b > 0) {if (b & 1) {c = a * c%mod;}a = a * a%mod; b >>= 1;}return c;}
void PRI(ll n) {bool a[n + 1LL]; for (int i = 0; i < n + 1LL; i++) {a[i] = 1LL;}for (int i = 2; i < n + 1LL; i++) {if (a[i]) {prime.pb(i); ll b = i; while (b <= n) {a[b] = 0; b += i;}}}}
template <typename T> T chmin(T& a, T b) {if(a>b)a=b;return a;}
template <typename T> T chmax(T& a, T b) {if(a<b)a=b;return b;}
bool isSqrt(ll a) {return pow(sqrt(a),2) == a ? 1 : 0;}
void YesNo(bool a) {if (a) cout << "Yes"; else cout << "No"; cout << endl;}
void yesno(bool a) {if (a) cout << "yes"; else cout << "no"; cout << endl;}
void YESNO(bool a) {if (a) cout << "YES"; else cout << "NO"; cout << endl;}
double dis(int x1, int x2, int y1, int y2) {
    return sqrt((double)abs(x1-x2)*(double)abs(x1-x2)+(double)abs(y1-y2)*(double)abs(y1-y2));
}

class topological {
public:
    bool uniq = true, can = true;
    int maxlen = 0;
    int n;
    queue<int> cnt;
    vector<int> in;
    vector<int> len;
    vector<vector<int>> loute;
    vector<int> res;

    topological (vector<vector<int>> lt) {
        loute = lt;
        n = loute.size();
        in = vector<int>(n);
        len = vector<int>(n);
        for (auto&& from : loute) {
            for (auto&& to : from) {
                in[to]++;
            }
        }
        for (int i = 0; i < n; i++) {
            if (!in[i]) {
                cnt.push(i);
            }
        }
    }

    void run() {
        while (!cnt.empty()) {
            int nxt = cnt.front(); cnt.pop();
            if (cnt.size()) uniq = false;
            res.pb(nxt);
            for (auto&& to : loute[nxt]) {
                --in[to];
                if (!in[to]) cnt.push(to);
                len[to] = max(len[to], len[nxt] + 1);
            }
        }
        if (res.size() != n) can = false;
        for (int i = 0; i < n; i++) {
            maxlen = max(maxlen, len[i]);
        }
    }
};

void solve() {
    int n, m; cin >> n >> m;
    vvi lt = vvi(n);
    rep (i, m) {
        int x, y; cin >> x >> y; --x, --y;
        lt[x].pb(y);
    }
    topological topo(lt);
    topo.run();
    cout << topo.maxlen << endl;
}

signed main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
}


Measured memory_bytes: 18176