//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define int long long
#define ll long long
using ull = unsigned long long;
using namespace std;
#define dump(x)                             \
    if (dbg) {                              \
        cerr << #x << " = " << (x) << endl; \
    }
#define overload4(_1, _2, _3, _4, name, ...) name
#define FOR1(n) for (ll i = 0; i < (n); ++i)
#define FOR2(i, n) for (ll i = 0; i < (n); ++i)
#define FOR3(i, a, b) for (ll i = (a); i < (b); ++i)
#define FOR4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define FOR(...) overload4(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define FORR(i, a, b) for (int i = (a); i <= (b); ++i)
#define bit(n, k) (((n) >> (k)) & 1) /*nのk bit目*/
namespace mydef {
const int INF = 1ll << 60;
const int MOD = 1e9 + 7;
template <class T>
bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return 1;
    } else
        return 0;
}
template <class T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return 1;
    } else
        return 0;
}
void Yes(bool flag = true) {
    if (flag)
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
}
void No(bool flag = true) {
    Yes(!flag);
}
void YES(bool flag = true) {
    if (flag)
        cout << "YES" << endl;
    else
        cout << "NO" << endl;
}
void NO(bool flag = true) {
    YES(!flag);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T& val) {
    std::fill((T*)array, (T*)(array + N), val);
}
bool dbg = true;
}  // namespace mydef
using namespace mydef;
#define pb push_back
//#define mp make_pair
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(), (v).end()
#define SZ(x) ((int)(x).size())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vp vector<pair<int, int>>
#define vvp vector<vector<pair<int, int>>>
#define pi pair<int, int>
//#define P pair<int, int>
//#define V vector<int>
//#define S set<int>
#define asn ans

#include <bits/stdc++.h>
using namespace std;
template <typename T>
struct edge {
    int to, id;
    T cost;
    edge(int to) : to(to), id(-1), cost(1) {}
    edge(int to, T cost) : to(to), id(-1), cost(cost) {}
    edge(int to, T cost, int id) : to(to), id(id), cost(cost) {}
    operator int() const { return to; }
    /*
        edge& operator=(const int& x) {
            to = x;
            return *this;
        }
        */
};
template <typename T>
class Graph : public vector<vector<edge<T>>> {
    //--------Basic--------
private:
    using Edge = edge<T>;
    int M;  //M:辺の数
    bool undirected = false;
    bool directed = false;
    bool unweighted = false;
    bool weighted = false;
    bool tree = false;
    //
public:
    //
public:
    Graph() = default;
    //unweighted
    void add_edge_undirected(int u, int v, int id = -1) {
        assert(!directed);
        assert(!weighted);
        undirected = true;
        unweighted = true;
        (*this)[u].emplace_back(Edge{v, 1, id});
        (*this)[v].emplace_back(Edge{u, 1, id});
    }
    void build_undirected(int m) {
        assert(!(*this).empty());
        assert(!directed);
        assert(!weighted);
        undirected = true;
        unweighted = true;
        M = m;
        for (int i = 0; i < m; i++) {
            int u, v;
            cin >> u >> v;
            u--;
            v--;
            (*this)[u].emplace_back(Edge{v, 1, i});
            (*this)[v].emplace_back(Edge{u, 1, i});
        }
    }
    void build_undirected(int n, int m) {
        (*this).resize(n);
        build_undirected(m);
    }
    void add_edge_directed(int u, int v, int id = -1) {
        assert(!undirected);
        assert(!weighted);
        directed = true;
        unweighted = true;
        (*this)[u].emplace_back(Edge{v, 1, id});
    }
    void build_directed(int m) {
        assert(!(*this).empty());
        assert(!undirected);
        assert(!weighted);
        directed = true;
        unweighted = true;
        M = m;
        for (int i = 0; i < m; i++) {
            int u, v;
            cin >> u >> v;
            u--;
            v--;
            (*this)[u].emplace_back(Edge{v, 1, i});
        }
    }
    void build_directed(int n, int m) {
        (*this).resize(n);
        build_directed(m);
    }
    //weighed
    void add_edge_undirected_weighed(int u, int v, T cost, int id = -1) {
        assert(!directed);
        assert(!unweighted);
        undirected = true;
        weighted = true;
        (*this)[u].emplace_back(Edge{v, cost, id});
        (*this)[v].emplace_back(Edge{u, cost, id});
    }
    void build_undirected_weighted(int m) {
        assert(!(*this).empty());
        assert(!directed);
        assert(!unweighted);
        undirected = true;
        weighted = true;
        M = m;
        for (int i = 0; i < m; i++) {
            int u, v;
            T cost;
            cin >> u >> v >> cost;
            u--;
            v--;
            (*this)[u].emplace_back(Edge{v, cost, i});
            (*this)[v].emplace_back(Edge{u, cost, i});
        }
    }
    void build_undirected_weighted(int n, int m) {
        (*this).resize(n);
        build_undirected_weighted(m);
    }
    void add_edge_directed_weighted(int u, int v, T cost, int id = -1) {
        assert(!undirected);
        assert(!unweighted);
        directed = true;
        weighted = true;
        (*this)[u].emplace_back(Edge{v, cost, id});
    }
    void build_directed_weighted(int m) {
        assert(!(*this).empty());
        assert(!undirected);
        assert(!unweighted);
        directed = true;
        weighted = true;
        M = m;
        for (int i = 0; i < m; i++) {
            int u, v;
            T cost;
            cin >> u >> v >> cost;
            u--;
            v--;
            (*this)[u].emplace_back(Edge{v, cost, i});
        }
    }
    void build_directed_weighted(int n, int m) {
        (*this).resize(n);
        build_undirected_weighted(m);
    }
    //tree
    void istree() {
        tree = true;
    }
    void build_tree(int n) {
        istree();
        build_undirected(n, n - 1);
    }
    void build_tree_weighted(int n) {
        istree();
        build_undirected_weighted(n, n - 1);
    }
    //print state
    void state() {
        cerr << endl
             << "Print State" << endl
             << "Edge :" << endl
             << "    Directed   : " << (directed ? "Yes" : "No") << endl
             << "    Undirected : " << (undirected ? "Yes" : "No") << endl
             << "    Weighted   : " << (weighted ? "Yes" : "No") << endl
             << "    Unweighted : " << (unweighted ? "Yes" : "No") << endl
             << "    Tree       : " << (tree ? "Yes" : "No") << endl
             << "Usable Functions : Graph" << endl
             << "    dijkstra(verified) : Yes" << endl
             << "    Warshall–Floyd(verified) : Yes" << endl
             << "    Kruskal(verified) : Yes" << endl
             << "    StronglyConnectedComponent(unverified): " << (StronglyConnectedComponent_ready ? "Yes" : "No") << endl
             << "Usable Funcitons : Tree" << endl
             << "    ReRooting(verified) : Yes" << endl;
    }
    //
    //--------reverse--------
public:
    void reverse() {
        int N = (*this).size();
        vector<pair<int, Edge>> V;
        V.reserve(M);
        for (int i = 0; i < N; i++) {
            for (auto& e : (*this)[i]) {
                V.emplace_back(make_pair(i, e));
            }
        }
        (*this).erase((*this).begin(), (*this).end());
        (*this).resize(N);
        for (auto& p : V)
            (*this)[p.second.to].emplace_back(Edge{p.first, p.second.cost, p.second.id});
    }
    //
    //--------Dijkstra--------
    //verified
public:
    //pair<vector<T>, vector<int>> Dijkstra(int s) {
    vector<T> Dijkstra(int s) {
        const T INF = numeric_limits<T>::max();
        using P = pair<T, int>;
        int N = (*this).size();
        vector<T> dist(N, INF);
        //vector<int> bef(N, -1);
        priority_queue<P, vector<P>, greater<P>> que;
        dist[s] = 0;
        que.emplace(dist[s], s);
        while (!que.empty()) {
            P p = que.top();
            que.pop();
            int now = p.second;
            if (dist[now] < p.first)
                continue;
            for (auto& p : (*this)[now]) {
                int nxt = p.to;
                T cost = p.cost;
                if (dist[nxt] > dist[now] + cost) {
                    dist[nxt] = dist[now] + cost;
                    //bef[nxt] = now;
                    que.emplace(dist[nxt], nxt);
                }
            }
        }
        return dist;
        //return make_pair(dist, bef);
    }
    //
    //
    //--------Warshall–Floyd--------
    //verified
public:
    vector<vector<T>> WarshallFloyd() {
        int N = (*this).size();
        const T INF = numeric_limits<T>::max() / 3;
        vector<vector<T>> ret(N, vector<T>(N, INF));
        for (int i = 0; i < N; i++) {
            for (auto& e : (*this)[i]) {
                ret[i][e.to] = e.cost;
            }
        }
        for (int k = 0; k < N; k++) {
            for (int i = 0; i < N; i++) {
                for (int j = 0; j < N; j++) {
                    if (ret[i][j] > ret[i][k] + ret[k][j])
                        ret[i][j] = ret[i][k] + ret[k][j];
                }
            }
        }
        return ret;
    }
    //
    //
    //--------Kruskal--------
    //verified
private:
    vector<int> Kruskal_data;
    bool Kruskal_unite(int x, int y) {
        x = Kruskal_root(x), y = Kruskal_root(y);
        if (x == y)
            return (false);
        if (Kruskal_data[x] > Kruskal_data[y])
            swap(x, y);
        Kruskal_data[x] += Kruskal_data[y];
        Kruskal_data[y] = x;
        return (true);
    }
    int Kruskal_root(int k) {
        if (Kruskal_data[k] < 0)
            return (k);
        return (Kruskal_data[k] = Kruskal_root(Kruskal_data[k]));
    }
    bool Kruskal_same(int x, int y) {
        return Kruskal_root(x) == Kruskal_root(y);
    }
    int Kruskal_size(int k) {
        return (-Kruskal_data[Kruskal_root(k)]);
    }
    //
public:
    template <class Compare = less<T>>
    pair<T, vector<bool>> Kruskal(bool flag = false) {
        struct edge2 {
            int from, to;
            T cost;
            bool used;
            int id;
            edge2(int from, int to, T cost, int id) : from(from), to(to), cost(cost), used(false), id(id) {}
        };
        vector<edge2> edges;
        int N = (*this).size();
        Kruskal_data.assign(N, -1);
        for (int i = 0; i < (int)(*this).size(); i++) {
            auto& V = (*this)[i];
            for (auto& e : V) {
                edges.emplace_back(i, e.to, e.cost, e.id);
            }
        }
        sort(edges.begin(), edges.end(), [](const edge2& a, const edge2& b) {
            return Compare()(a.cost, b.cost);
        });
        T ret = 0;
        vector<bool> V;
        if (flag)
            V.resize(M, false);
        for (auto& e : edges) {
            if (Kruskal_unite(e.from, e.to)) {
                ret += e.cost;
                e.used = true;
            }
        }
        if (flag)
            for (auto& e : edges) {
                assert(e.id >= 0 && e.id < M);
                if (e.used)
                    V[e.id] = true;
            }
        if (N == Kruskal_size(0))
            return make_pair(ret, V);
        else
            return make_pair(-1, V);
    }
    //
    //--------StronglyConnectedComponent--------
    //Unverified
public:
    vector<vector<int>> SCC_R, SCC_T;
    //
private:
    bool StronglyConnectedComponent_ready = false;
    vector<int> SCC_cmp, SCC_ord;
    vector<bool> SCC_used;
    //
public:
    int Gbuild_SCC() {
        cerr << "Please Verify" << endl;
        assert(!undirected);
        assert(directed);
        int N = (*this).size();
        SCC_R.resize(N);
        for (int i = 0; i < N; i++) {
            for (int& x : (*this)[i]) {
                SCC_R[x].emplace_back(i);
            }
        }
        SCC_cmp.resize(N);
        fill(SCC_cmp.begin(), SCC_cmp.end(), -1);
        SCC_used.resize(N);
        fill(SCC_used.begin(), SCC_used.end(), false);
        StronglyConnectedComponent_ready = true;
        return build_StronglyConnectedComponent_init();
    }
    int SCC(int k) {
        assert(StronglyConnectedComponent_ready);
        return SCC_cmp[k];
    }
    //
private:
    void build_StronglyConnectedComponent_dfs(int now) {
        if (SCC_used[now])
            return;
        SCC_used[now] = true;
        for (auto nxt : (*this)[now])
            build_StronglyConnectedComponent_dfs(nxt);
        SCC_ord.emplace_back(now);
    }
    void build_StronglyConnectedComponent_rdfs(int now, int count) {
        if (SCC_cmp[now] != -1)
            return;
        SCC_cmp[now] = count;
        for (auto to : SCC_R[now])
            build_StronglyConnectedComponent_rdfs(to, count);
    }
    int build_StronglyConnectedComponent_init() {
        int n = (int)(*this).size();
        for (int i = 0; i < n; i++)
            build_StronglyConnectedComponent_dfs(i);
        reverse(SCC_ord.begin(), SCC_ord.end());
        int group = 0;
        for (auto& i : SCC_ord) {
            if (SCC_cmp[i] == -1) {
                build_StronglyConnectedComponent_rdfs(i, group);
                group++;
            }
        }
        SCC_T.resize(group);
        for (int i = 0; i < n; i++) {
            for (auto& to : (*this)[i]) {
                int s = SCC_cmp[i], t = SCC_cmp[to];
                if (s != t)
                    SCC_T[s].emplace_back(t);
            }
        }
        return group;
    }
    //
    //
    //
    //--------TopologicalSort--------
    //未実装
    //
    //
    //--------Tree--------
    //
    //
    //--------ReRooting--------
public:
    template <typename sum_t>
    pair<vector<sum_t>, vector<vector<sum_t>>>
    Tbuild_ReRooting(
        const function<sum_t(sum_t, sum_t)> f,
        const function<sum_t(sum_t, Edge)> gg,
        sum_t ident) {
        assert(tree);
        int N = (*this).size();
        vector<sum_t> subdp(N, ident), dp(N, ident);
        vector<vector<sum_t>> g_dp(N), g_ndp(N), memo(N);
        vector<vector<bool>> seen(N);
        for (int i = 0; i < N; i++) {
            int S = (*this)[i].size();
            g_dp[i].resize(S, ident);
            g_ndp[i].resize(S, ident);
            memo[i].resize(S);
            seen[i].resize(S, false);
        }
        stack<int> stk;
        stk.push(0);
        vector<int> par(N), ord(N);
        int index = 0;
        par[0] = -1;
        while (!stk.empty()) {
            int node = stk.top();
            stk.pop();
            ord[index++] = node;
            for (auto& e : (*this)[node]) {
                if (e.to == par[node])
                    continue;
                stk.push(e.to);
                par[e.to] = node;
            }
        }
        for (int k = ord.size() - 1; k >= 0; k--) {
            int idx = ord[k];
            for (int i = 0; i < (int)((*this)[idx].size()); i++) {
                auto& e = (*this)[idx][i];
                if (e.to == par[idx])
                    continue;
                if (!seen[idx][i]) {
                    memo[idx][i] = gg(subdp[e.to], e);
                    seen[idx][i] = true;
                }
                subdp[idx] = f(subdp[idx], memo[idx][i]);
            }
        }
        vector<sum_t> top(N, ident);
        for (int k = 0; k < (int)(ord.size()); k++) {
            int idx = ord[k];
            sum_t buff{ident};
            for (int i = 0; i < (int)((*this)[idx].size()); i++) {
                auto& e = (*this)[idx][i];
                g_ndp[idx][i] = buff;
                if (!seen[idx][i]) {
                    memo[idx][i] = gg(par[idx] == e.to ? top[idx] : subdp[e.to], e);
                    seen[idx][i] = true;
                }
                g_dp[idx][i] = memo[idx][i];
                buff = f(buff, g_dp[idx][i]);
            }
            dp[idx] = buff;
            buff = ident;
            for (int i = (*this)[idx].size() - 1; i >= 0; i--) {
                auto& e = (*this)[idx][i];
                if (e.to != par[idx])
                    top[e.to] = f(g_ndp[idx][i], buff);
                g_ndp[idx][i] = f(g_ndp[idx][i], buff);
                buff = f(buff, g_dp[idx][i]);
            }
        }
        return make_pair(dp, memo);
    }
    //
    //
    //
    //TODO:lca,centroid,diameter,hl-decomposition
};
//グローバルでの使用のみ想定
Graph<int> G;
int N, M;

int memo[101010];

int f(int x) {
    if (memo[x])
        return memo[x];
    int ret = 0;
    for (auto& nxt : G[x])
        chmax(ret, f(nxt) + 1);
    return memo[x] = ret;
}

void solve() {
    G.reverse();
    int ans = 0;
    for (int i = 0; i < N; i++) {
        chmax(ans, f(i));
    }
    cout << ans << endl;
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    cin >> N >> M;
    G.build_directed(N, M);

    solve();
    return 0;
}


Measured memory_bytes: 14096