#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
//#include "optimization.h"

using namespace std;

vector<int> used;

void dfs(int v, vector<vector<int> > &g) {
    for (int u : g[v]) {
        if (!used[u]) {
            dfs(u, g);
        }
        used[v] = max(used[v], used[u]);
    }
    ++used[v];
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int n, m;
    cin >> n >> m;
    vector<vector<int> > v(n);
    for (int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        --a;
        --b;
        v[a].push_back(b);
    }
    used.resize(n);
    for (int i = 0; i < n; ++i) {
        if (!used[i]) {
            dfs(i, v);
        }
    }
    cout << *max_element(used.begin(), used.end()) - 1;
}

/*
5 5
 1 2
 2 3
 3 4
 3 5
 1 5
 */

/*
 4 2 1 1 4 2
 ..*.
 *...
 */