include <bits/stdc++.h>

using namespace std; const int maxN = 1e6 + 5; const int oo = 1e18 + 7; const int MOD = 1e9 + 7; const int LOG = 65; const int MAXBIT = 31; int N, M, adj[maxN]; vector <int> aaa[maxN]; int DFS(int u) { if (adj[u] != 0) { return adj[u]; } int ma = 0; for (auto v : aaa[u]) { ma = max(ma, DFS(v) + 1); } return adj[u] = ma; } main() { iosbase::syncwithstdio(0); cin.tie(0); cout.tie(0); int n,m; cin >> n >> m; for (int i = 1, u, v; i <= m; i++) { cin >> u >> v; aaa[u].push_back(v); } int ma = 0; for (int u = 1; u <= n; ++u) { ma = max(ma, DFS(u)); } cout << ma << '\n'; return 0; }