G.cpp

/*
 © 2020-09-23 17:26:40 Franco1010 All Rights Reserved
*/

#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
#pragma GCC target("avx,avx2,fma")
#include <bits/stdc++.h>
#define f first
#define s second
#define fore(i,a,b) for(int i = (a), ThxMK = (b); i < ThxMK; ++i)
#define pb push_back
#define all(s) begin(s), end(s)
#define _ ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define sz(s) int(s.size())
#define ENDL '\n'
using namespace std;
typedef long double ld;
typedef long long lli;
typedef pair<lli,lli> ii;
typedef vector<lli> vi;
#define deb(x) cout << #x": " << (x) << endl;
const int N = 1e5 + 5;
vector<lli> g[N];
lli dp[N];
bool vis[N];
lli dfs(int u){
  lli &res = dp[u];
  if(!vis[u]){
    vis[u] = true;
    res = 0;
    for(auto v : g[u]){
      res = max(res, dfs(v) + 1ll);
    }
  }
  return res;
}
int main(){ _
  lli n, m; cin >> n >> m;
  fore(i, 0, m){
    lli x, y; cin >> x >> y;
    g[x].pb(y);
  }
  lli res = 0;
  fore(i, 1, n + 1) res = max(res, dfs(i));
  cout << res << ENDL;
  return 0;
}