#include<bits/stdc++.h>
                            using namespace std;

                            #define FOR(i,a,b) for(int i = (a); i < (b); i++)
                            #define rep(i,n) FOR(i,0,n)
                            #define RFOR(i,b,a) for(int i = (b) - 1; i>= (a); i--)
                            #define ITER(it, a) for(typeof(a.begin()) it = a.begin(); it != a.end(); it++)
                            #define FILL(a, value) memset(a, value, sizeof(a)

                            #define SZ(a) (int)a.size()
                            #define ALL(a) a.begin(), a.end()
                            #define PB push_back
                            #define MP make_pair
                            #define er erase
                            #define in insert
                            #define f first
                            #define s second



                            typedef  long long LL;
                            typedef vector<int> VI;
                            typedef vector<LL > VL;
                            typedef vector<vector<int > > VVI;
                            typedef vector<vector<LL>> VVL;
                            typedef vector <vector<pair<int, int >> >VVPI;
                            typedef vector<vector<pair<LL, LL >> >VVPL;
                            typedef vector<pair<LL, LL >> PL;
                            typedef vector<PL> VPL;
                            typedef pair<int, int> PII;
                            typedef vector<PII> VPI;

                            const double PI = acos(-1.0);
                            const int INF = 1e9;
                            const LL LINF = (LL)1e18;

                            const double EPS = 1e-7;
                            const int MAS =  3 *1e5 + 10;
                            const int mod = 998244353;
                            const int MOD =1e9+7;
                            const int LOG= 19;
                            const int ar = 1e5 +100;
                            const int MAX = 1e6 + 1e2;

                            bool used[ar];
                            int dp[ar];
                             vector<vector<int >> g;
                            int dfs(int v)
                            {
                                used[v] = true;
                                for(auto to :g[v])
                                {

                                    if(!used[to] )dfs(to);
                                    dp[v] = max(dp[v],dp[to] + 1);
                                }
                             //   cout << v << ' ' << dp[v] << endl;
                                return dp[v];
                            }
                        int main() {
                            ios_base::sync_with_stdio(false);
                            cin.tie(NULL);
                           int n,m;
                           cin >> n >> m ;
                           g.resize(n);
                           rep(i,m)
                           {
                               int x,y;
                               cin >> x >> y;
                               --x,--y;
                               g[x].PB(y);
                           }
                           rep(i,n)
                           {
                          //     cout << '*' << g[i].size() << ' ' << used[i] << endl;
                               if(g[i].size() && !used[i])dfs(i);
                           }
                           cout << *max_element(dp,dp + n);



                        }

















































Measured memory_bytes: 8320