#include <bits/stdc++.h>

using namespace std;
using ll = long long int;
constexpr ll M = 1e9 + 7;
inline ll sum_1(const ll n) { return ((n * (n + 1)) / 2) % M; }
inline ll sum_2(const ll n) { return ((n * (n + 1) * (2 * n + 1)) / 6) % M; }
inline ll sum_3(const ll n) { return (((n * (n + 1)) / 2) % M * ((n * (n + 1)) / 2) % M) % M; }
inline ll sum_1(const ll i, const ll j) { return (sum_1(j) - sum_1(i - 1) + M) % M; }
inline ll sum_2(const ll i, const ll j) { return (sum_2(j) - sum_2(i - 1) + M) % M; }
inline ll sum_3(const ll i, const ll j) { return (sum_3(j) - sum_3(i - 1) + M) % M; }

/* clang-format off */
namespace myDebug {
template <class T> auto is_printable(T *v) -> decltype(cout << *v, 0);
template <class T> char is_printable(...);
template <class T> struct range{T begin, end;};

class Debug {
    class DebugImpl {
      public:
        ~DebugImpl() { cout << endl; }
        #define enable_cond(cmp) enable_if<sizeof(is_printable<T>(nullptr)) cmp 1, DebugImpl &>::type
        template <class T> typename enable_cond(!=) operator<<(const T v) {
            cout << v; return *this;
        }
        template <class T> typename enable_cond(==) operator<<(const T v) {
            return *this << range<decltype(begin(v))>({begin(v), end(v)});
        }
        template <class T, class U> DebugImpl& operator<<(const pair<T, U> p) {
            return *this << "(" << p.first << ", " << p.second << ")";
        }
        template <class T> DebugImpl& operator<<(const range<T> v) {
            for (auto it = v.begin; it != v.end; ++it) {
                *this << *it << " ";
            }
            return *this << "\n";
        }
        #ifdef enable_cond
        #undef enable_cond
        #endif
    } os;
  public:
    #ifdef D_LOCAL
    template<class T> Debug& operator<<(const T v) {
        os << v; cout << endl; return *this;
    }
    #else
    template <class T> Debug& operator<<(const T &) { return *this; }
    #endif
};
#define px(...) #__VA_ARGS__":" << (__VA_ARGS__)
}; // namespace myDebug
/* clang-format on */
myDebug::Debug debug;

ll pow_f(ll b, ll e, ll M = ::M) {
    ll ans = 1;
    b = b % M;
    while (e) {
        if (e & 1) {
            ans = (ans * b) % M;
        }
        b = (b * b) % M;
        e >>= 1;
    }
    return ans;
}

ll nCr(ll n, ll r) {
    if (r > n || n < 0 || r < 0) { return 0; }
    if (n == r || r == 0) { return 1; }
    r = min(r, n - r);
    ll ans = 1;
    for (ll i = 1; i <= r; ++i) {
        ans = (ans * (n - i + 1)) / (i);
    }
    return ans;
}

ll nCr(tuple<ll, ll> t) {
    auto n = get<0>(t);
    auto k = get<1>(t);
    return nCr(n + k - 1, k);
}

ll modinv(ll x, ll M = ::M) {
    return pow_f(x, M - 2, M);
}

struct graph {
    ll n, m;
    vector<vector<ll>> adj;
    vector<ll> dp;
    graph(ll n, ll m) : n(n), m(m), adj(n+1), dp(n+1, 0) {}

    void addEdge(ll u, ll v) {
        adj[u].push_back(v);
    }

    ll dfsutil(ll root, vector<bool> &visited) {
        visited[root] = true;
        ll max_path = 0;
        for (auto &i : adj[root]) {
            if (!visited[i]) {
                auto d = dfsutil(i, visited);
                max_path = max(max_path, 1LL + d);
            } else {
                max_path = max(max_path, 1LL + dp[i]);
            }
        }
        dp[root] = max(dp[root],max_path);
        return dp[root];
    }

    ll dfs() {
        vector<bool> visited(n+1, false);

        for (ll i = 1; i <= n; ++i) {
            if (!visited[i]) {
                dfsutil(i, visited);
            }
        }
        return *max_element(dp.begin(), dp.end());
    }
};


void solve() {
    ll n, m;
    cin >> n >> m;
    graph g(n, m);
    while (m--) {
        ll u, v;
        cin >> u >> v;
        g.addEdge(u, v);
    }
    cout << g.dfs() << endl;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    #ifdef D_LOCAL
        freopen("in.in", "r", stdin);
        freopen("out.out", "w", stdout);
    #endif

    solve();

    return 0;
}

Measured memory_bytes: 8192