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

/*BigInteger
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/rational.hpp>
namespace xxx = boost::multiprecision;
using Bint = xxx::cpp_int;
using Real = xxx::number<xxx::cpp_dec_float<1024>>;
*/

#define int long long
#define pb(x) push_back(x)
#define m0(x) memset((x), 0LL, sizeof(x))
#define mm(x) memset((x), -1LL, sizeof(x))

//container
#define ALL(x) (x).begin(), (x).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define EACH(i, c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define UNIQUE(v) (v).erase(unique((v).begin(), (v).end()), (v).end());
#define PERM(c) \
  sort(ALL(c)); \
  for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))

// debug
#define GET_VAR_NAME(variable) #variable
#define test(x) cout << GET_VAR_NAME(x) << " = " << x << endl;

// bit_macro
#define Bit(n) (1LL << (n))
#define Bitset(a, b) (a) |= (1LL << (b))
#define Bitunset(a, b) (a) &= ~(1LL << (b))
#define Bitcheck(a, b) ((((a) >> (b)) & 1LL) == 1LL)
#define Bitcount(a) __builtin_popcountll((a))

//typedef
typedef long long lint;
typedef unsigned long long ull;
typedef complex<long double> Complex;
typedef pair<int, int> P;
typedef tuple<int, int, int> TP;
typedef vector<int> vec;
typedef vector<vec> mat;

//constant
constexpr int INF = (int)1e18;
constexpr int MOD = (int)1e9 + 7;
constexpr double PI = (double)acos(-1);
constexpr double EPS = (double)1e-10;
constexpr int dx[] = {-1, 0, 0, 1, 0, -1, -1, 1, 1};
constexpr int dy[] = {0, -1, 1, 0, 0, 1, -1, 1, -1};

//
template <typename T>
void chmax(T &a, T b) { a = max(a, b); }
template <typename T>
void chmin(T &a, T b) { a = min(a, b); }
//
inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T>
inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}

//
struct Accelerate_Cin {
  Accelerate_Cin() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(20);
  };
};

int M;

//トポロジカルソート
const int MAX = 101010;

vector<int> G[MAX];
list<int> out;  // sortされた頂点が左から順番に格納される。
bool V[MAX];    //訪問したかどうか
int indeg[MAX]; //入次数
int N;          //頂点数

void bfs(int s) {
  queue<int> q;
  q.push(s);
  V[s] = true;
  while (!q.empty()) {
    int u = q.front();
    q.pop();
    out.push_back(u);
    for (auto v : G[u]) {
      indeg[v]--;
      if (indeg[v] == 0 && V[v] != true) {
        V[v] = true;
        q.push(v);
      }
    }
  }
}

void topoSort() {
  for (int i = 0; i < N; i++) {
    indeg[i] = 0;
  }

  for (int i = 0; i < N; i++) {
    for (auto v : G[i]) {
      indeg[v]++;
    }
  }

  for (int i = 0; i < N; i++) {
    if (indeg[i] == 0 && V[i] != true) {
      bfs(i);
    }
  }
}

signed main() {
  cin >> N >> M;
  for (int i = 0; i < M; i++) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    G[x].push_back(y);
  }

  topoSort();

  static int dp[101010];
  m0(dp);

  int ans = 0;
  for (auto x : out) {
    for (auto y : G[x]) {
      dp[y] = max(dp[y], dp[x] + 1);
      chmax(ans, dp[y]);
    }
  }

  cout << ans << endl;
  return 0;
}


Measured memory_bytes: 10496