#include <algorithm>
#include <iostream>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using ll = long long;
using namespace std;
static ll imin = std::numeric_limits<ll>::min() / 10;
static ll imax = std::numeric_limits<ll>::max() / 10;
#define rep(i, n) for (ll i = 0; (i) < (n); ++(i))
#define rep1(i, n) for (ll i = 1; (i) < (n); ++(i))
#define rev_rep(i, n) for (ll i = (n)-1; (i) >= 0; --(i))
#define range(a, i, b) for (ll i = (a); (i) < (b); ++(i))

#ifdef LOCAL
#define dump(x) cout << #x << ":" << x << "\n";
#endif
#ifndef LOCAL
#define dump(x)
#endif

#define all(v) v.begin(), v.end()
#define readl(N)                                                               \
  ll N;                                                                        \
  cin >> N;
#define readlv(N, vec)                                                         \
  vector<ll> vec(N);                                                           \
  rep(i, N) { cin >> vec[i]; }
#define greater_than [](ll l, ll r) { return l > r; }
template <class T1, class T2>
std::ostream &operator<<(std::ostream &os, std::map<T1, T2> &mp);
template <class T1, class T2>
std::ostream &operator<<(std::ostream &os, std::pair<T1, T2> &pair);
template <class T>
std::ostream &operator<<(std::ostream &os, std::vector<T> &vec);
template <class T1, class T2>
std::ostream &operator<<(std::ostream &os, std::pair<T1, T2> &pair) {
  os << '(' << pair.first << ',' << pair.second << ')';
  return os;
}

template <class T>
std::ostream &operator<<(std::ostream &os, std::vector<T> &vec) {
  std::size_t n = vec.size();
  if (n == 0) {
    return os;
  }
  os << " ";
  for (ll i = 0; i < n - 1; ++i) {
    os << vec[i] << " ";
  }
  os << vec[n - 1];
  os << std::endl;
  return os;
}

template <class T1, class T2>
std::ostream &operator<<(std::ostream &os, std::map<T1, T2> &mp) {
  std::size_t n = mp.size();
  if (n == 0) {
    return os;
  }
  os << " ";
  for (auto &&x : mp) {
    os << x << " ";
  }
  os << std::endl;
  return os;
}

template <class T> std::ostream &operator<<(std::ostream &os, std::set<T> &mp) {
  std::size_t n = mp.size();
  if (n == 0) {
    return os;
  }
  os << " ";
  for (auto &&x : mp) {
    os << x << " ";
  }
  os << std::endl;
  return os;
}

template <class T> inline bool chmin(T &a, T const b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T const b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

ll fact(ll n) {
  ll res = 1;
  while (n > 0) {
    res *= n;
    n--;
  }
  return res;
}

ll pow(ll i, ll n) {
  ll res = 1;
  while (n >= 0) {
    res *= i;
    n--;
  }
  return res;
}

ll gcd(ll l, ll r) {
  if (l < r) {
    return gcd(r, l);
  }
  if (r == 0) {
    return l;
  }
  return gcd(r, l % r);
}

template <class T> vector<T> accum_sum(vector<T> const &a) {
  vector<T> b(a.size() + 1);
  b[0] = 0;
  rep(i, a.size()) { b[i + 1] = b[i] + a[i]; }
  return b;
}
constexpr ll mod = 1e9 + 7;

vector<vector<ll>> G;
vector<ll> dp;
ll rec(ll n) {
  if (dp[n] != -1) {
    return dp[n];
  } else {
    ll res = 0;
    for (auto &&nv : G[n]) {
      chmax(res, rec(nv) + 1);
    }
    return dp[n] = res;
  }
}

int main(int argc, char const *argv[]) {
  ll N, M;
  cin >> N >> M;

  G = vector<vector<ll>>(N);
  dp = vector<ll>(100100, -1);

  rep(i, M) {
    ll x, y;
    cin >> x >> y;
    --x;
    --y;
    G[x].push_back(y);
  }
  ll res = 0;
  rep(v, N) { chmax(res, rec(v)); }

  cout << res << endl;
  return 0;
}


Measured memory_bytes: 8576