#include <bits/stdc++.h>
using namespace std;
 
template<typename T> istream &operator>>(istream &is, vector<T> &vec){ for (auto &v : vec) is >> v; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec){ os << "["; for (auto v : vec) os << v << ", "; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ", "; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ", "; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ", "; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ", "; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ", "; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << ", " << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << ": " << v.second << ", "; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << ": " << v.second << ", "; os << "}"; return os; }
template<typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template<typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template<typename T> bool chmax(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool chmin(T &m, const T q) { if (q < m) {m = q; return true;} else return false; }
template<typename T1, typename T2> pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template<typename T1, typename T2> pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
#define f first
#define s second
int vertices, edges;
vector<int> memo;
vector<vector<int>> adj;

int DFS(int node) {
    if (adj[node].empty()) {
        memo[node] = 0;
        return 0;
    }
    if(memo[node] >= 0) {
        return memo[node];
    }
    for(int i : adj[node]) {
        memo[node] = max(memo[node], DFS(i));
    }
    memo[node]++;
    return memo[node];
}


int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cin >> vertices >> edges;
    memo.resize(vertices + 1, -1);
    adj.resize(vertices + 1);
    for(int i = 0; i < edges; i++) {
        int a, b;
        cin >> a >> b;
        adj[a].push_back(b);
    }
    for(int i = 1; i <= vertices; i++) {
        DFS(i);
    }
    int ans = 0;
    for(int i = 1; i <= vertices; i++) {
        if(memo[i] > ans) ans = memo[i];
    }
    cout << ans;
}



Measured memory_bytes: 10984