#include <bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef long long ll;
typedef long double lld;
typedef long long int lli;
using namespace std;
const int N = 1000001;
const int MOD=1e9+7;
const bool DEBUG = 1;
#define sd(x) scanf("%d", &x)
#define sd2(x, y) scanf("%d%d", &x, &y)
#define sd3(x, y, z) scanf("%d%d%d", &x, &y, &z)
#define endl "\n"
#define fi first
#define se second
#define eb emplace_back
#define fbo find_by_order
#define ook order_of_key
#define pb(x) push_back(x)
#define mp(x, y) make_pair(x, y)
#define LET(x, a) __typeof(a) x(a)
#define foreach(it, v) for (LET(it, v.begin()); it != v.end(); it++)
#define MEMS(a, b) memset(a, b, sizeof(a))
#define _			\
    ios_base::sync_with_stdio(false);			\
    cin.tie(NULL);			\
    cout.tie(NULL);
#define __ 			\
    freopen("input.txt", "r", stdin);			\
    freopen("output.txt", "w", stdout);
#define all(c) c.begin(), c.end()
#define inf 1000000000000000001
#define epsilon 1e-6
#define int ll
#define RUN_T			 \
    int _t; 			 \
    cin >> _t;			 \
    while (_t--)			 
#define tr(...)				\
if (DEBUG) {				\
cout << __FUNCTION__ << ' ' << __LINE__ << " = ";				\
trace(#__VA_ARGS__, __VA_ARGS__);				\
}
template <typename S, typename T>
ostream &operator<<(ostream &out, pair<S, T> const &p) {
    out << '(' << p.fi << ", " << p.se << ')';
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, set<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T, typename V>
ostream &operator<<(ostream &out, map<T, V> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << "\n" << (i->first) <<  ":"  << (i->second);
    return out;
}
template <typename T, typename V>
ostream &operator<<(ostream &out, unordered_map<T, V> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << "\n" << (i->first) <<  ":"  << (i->second);
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, multiset<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, unordered_set<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, unordered_multiset<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T> ostream &operator<<(ostream &out, vector<T> const &v) {
    ll l = v.size();
    for (ll i = 0; i < l - 1; i++)
        out << v[i] << ' ';
    if (l > 0)
        out << v[l - 1];
    return out;
}
template <typename T> void trace(const char *name, T &&arg1) {
    cout << name <<  ":"  << arg1 << endl;
}
template <typename T, typename... Args>
void trace(const char *names, T &&arg1, Args &&... args) {
    const char *comma = strchr(names + 1, ',');
    cout.write(names, comma - names) <<  ":"  << arg1 <<  "|" ;
    trace(comma + 1, args...);
}
template <typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int gcd(int a, int b) {
     if (b == 0)
        return a;
    return gcd(b, a % b);
}
vector<vector<int>>g;
int topological_sort(int n) {
  queue<int> q;
  vector<int> indeg(n + 1, 0);
  for (int i = 1; i <= n; i++) {
    for (auto p : g[i]) {
      indeg[p]++;
    }
  }
  vector<int>dp(n+1,-1);
  for (int i = 1; i <= n; i++) {
    if(indeg[i] == 0) {
      q.push(i);
      dp[i] = 0;
    }
  }
  int ans = 0;
  while (!q.empty()) {
    int x = q.front();
    q.pop();
    for (auto y : g[x]) {
      indeg[y]--;
      if (indeg[y] == 0) {
        dp[y] = max(dp[x]+1,dp[y]);
        q.push(y);
      }
    }
  }
  int maxi = 0;
  for(int i = 1; i <= n; i++){
      maxi = max(maxi, dp[i]);
  }
  return maxi;
}
int32_t main() {
    _
    int n , m, x, y;
    cin >> n >> m; 
    g.resize(n+1);
    for(int i = 1; i <= m; i++){
        cin >> x >> y;
        g[x].pb(y); 
    }
    int ans = topological_sort(n);
    cout << ans << endl;
}

Measured memory_bytes: 7296