Problem p03166 (C++)

Buggy submission (status: Time Limit Exceeded):
/*
 * じょえチャンネル
 * 高評価・チャンネル登録よろしくおねがいします！
 * https://www.youtube.com/channel/UCRXsI3FL_kvaVL9zoolBfbQ
 */

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

// here!!!
//  define int long long !!!!!

#define int long long

// define int long long !!!!!

#define mod 1000000007ll
// constexpr int mod = 998244353ll;

constexpr double PI = 3.141592653589793238462643383279;
// constexpr double eps = DBL_EPSILON;

typedef long long ll;

#ifdef int
#define inf (int)(3e18)
#else
#define inf (int)(5e8)
#endif

#define intt long long
#define itn long long
#define innt long long
#define P pair<long long, long long>

#define rep(i, n) for (int i = 0; i < n; i++)
#define REP(i, n) for (int i = 1; i <= n; i++)
#define rev_rep(i, n) for (int i = n - 1; i >= 0; i--)
#define REV_REP(i, n) for (int i = n; i >= 1; i--)

#define ALL(v) v.begin(), v.end()

#define smallpriority_queue(T) priority_queue<T, vector<T>, greater<T>>

using namespace std;

// Library
// モッドパウ
inline int modpow(int x, int y, int m = mod) {
  int res = 1;
  while (y) {
    if (y & 1) {
      res *= x;
      res %= m;
    }
    x = x * x % m;
    y /= 2;
  }
  return res;
}

int mypow(int x, int y) {
  int res = 1;
  while (y) {
    if (y % 2) {
      res *= x;
    }
    x = x * x;
    y /= 2;
  }
  return res;
}
// is the number (x) a prime number?
bool prime(int x) {
  if (!x || x == 1) {
    return false;
  }
  for (int i = 2; i * i <= x; i++) {
    if (!(x % i)) {
      return false;
    }
  }
  return true;
}

// saidai-kouyakusuu
inline int gcd(int x, int y) {
  if (!y) {
    return x;
  }
  return gcd(y, x % y);
}

// number of keta
int keta(int x) {
  int ans = 0;
  while (x) {
    x /= 10;
    ans++;
  }
  return ans;
}

// number of 2shinsuu keta
int bitketa(int x) {
  int ans = 0;
  while (x) {
    x >>= 1;
    ans++;
  }
  return ans;
}

// sum of keta
int ketasum(int x) {
  int ans = 0;
  while (x) {
    ans += x % 10;
    x /= 10;
  }
  return ans;
}

inline int lcm(int x, int y) {
  int ans = x / gcd(x, y) * y;
  return ans;
}
int twobeki(int x) {
  int ans = 0;
  while (1) {
    if (!(x & 1)) {
      ans++;
      x >>= 1;
    } else {
      break;
    }
  }
  return ans;
}

template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
void Yes() { cout << "Yes" << endl; }
void No() { cout << "No" << endl; }
void YES() { cout << "YES" << endl; }
void NO() { cout << "NO" << endl; }

#define fin(i) scanf("%lld", &i)
#define fout(i) printf("%lld", i)
#define fendl printf("\n")

int kai(int x, int y) {
  int res = 1;
  for (int i = x - y + 1; i <= x; i++) {
    res *= i;
    res %= mod;
  }
  return res;
}

int comb(int x, int y) {
  if (y > x)
    return 0;
  //    cout<<kai(x, y)<<' '<<modpow(kai(y, y), mod - 2)<<endl;
  return kai(x, y) * modpow(kai(y, y), mod - 2) % mod;
}
// Library-End

// 入出力高速化時にはoff！！！！
// #define vecin(v) for(int i=0;i<v.size();i++)scanf("%lld",&v[i]);
// #define vecout(v) {if(v.size())printf("%lld",v[0]);for(int
// i=1;i<(int)v.size();i++)printf(" %lld",v[i]);printf("\n");}

template <class T> class SegTree {
  int n;
  vector<T> node;
  T def;
  function<T(T, T)> operation;
  function<T(T, T)> update;

public:
  SegTree(unsigned int _n, T _def, function<T(T, T)> _operation,
          function<T(T, T)> _update)
      : def(_def), operation(_operation), update(_update) {
    n = 1;
    while (n < _n) {
      n *= 2;
    }
    node = vector<T>(n * 2, def);
  }
  SegTree(vector<int> &initvec, function<T(T, T)> _operation,
          function<T(T, T)> _update)
      : operation(_operation), update(_update) {
    n = 1;
    while (n < initvec.size()) {
      n *= 2;
    }
    node = vector<T>(n * 2, def);
    for (int i = n; i < n + initvec.size(); i++) {
      node[i] = initvec[i - n];
    }
    for (int i = n - 1; i >= 1; i--) {
      node[i] = operation(node[i * 2], node[i * 2 + 1]);
    }
  }
  void change(int i, T x) {
    i += n;
    node[i] = update(node[i], x);
    while (i >= 1) {
      i >>= 1;
      node[i] = operation(node[i * 2], node[i * 2 + 1]);
    }
  }
  T query(int l, int r) {
    l += n;
    r += n;
    T rx = def, lx = def;
    while (l < r) {
      if (l & 1) {
        lx = operation(lx, node[l]);
        l++;
      }
      if (r & 1) {
        r--;
        rx = operation(node[r], rx);
      }
      l >>= 1;
      r >>= 1;
    }
    return operation(lx, rx);
  }
  T operator[](const int &x) { return node[x + n]; }
  void fill(T x) { std::fill(ALL(node), x); }
  void print() {
    rep(i, n) std::cout << operator[](i) << " ";
    std::cout << std::endl;
  }
  T queryForALL() { return node[1]; }
};

#define R_MIN ([](long long a, long long b) { return min(a, b); })
#define R_MAX ([](long long a, long long b) { return max(a, b); })
#define R_SUM ([](long long a, long long b) { return a + b; })

#define NORMAL_UPDATE ([](long long a, long long b) { return b; })
#define ADD_UPDATE ([](long long a, long long b) { return a + b; })
#define MINUS_UPDATE ([](long long a, long long b) { return a - b; }

class Union_Find {
  vector<int> par;
  vector<int> ookisa;

public:
  Union_Find(int size) {
    par = vector<int>(size);
    ookisa = vector<int>(size);
    for (int i = 0; i < size; i++) {
      par[i] = i;
      ookisa[i] = 1;
    }
  }

  int find(int x) {
    if (par[x] == x) {
      return x;
    }
    return par[x] = find(par[x]);
  }

  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y) {
      return;
    }
    if (ookisa[x] < ookisa[y]) {
      par[x] = y;
      ookisa[y] += ookisa[x];
      ookisa[x] = 0;
    } else {
      par[y] = x;
      ookisa[x] += ookisa[y];
      ookisa[y] = 0;
    }
  }
  int size(int i) {
    i = find(i);
    return ookisa[i];
  }
  bool same(int x, int y) { return find(x) == find(y); }
};

class BIT {
  vector<int> data;
  int size = 0;

public:
  BIT(int _size) {
    data = vector<int>(_size + 1);
    size = _size;
  }
  void add(int i, int x) {
    while (i <= size) {
      data[i] += x;
      i += i & -i;
    }
  }
  int sum(int i) {
    assert(i <= size);
    int s = 0;
    while (i > 0) {
      s += data[i];
      i -= i & -i;
    }
    return s;
  }
  int lower_bound(int x) {
    if (x <= 0) {
      return 0;
    } else {
      int i = 0;
      int r = 1;
      while (r < size)
        r = r << 1;
      for (int len = r; len > 0; len = len >> 1) {
        if (i + len < size && data[i + len] < x) {
          x -= data[i + len];
          i += len;
        }
      }
      return i + 1;
    }
  }
};

// Union-Find-End

int perm[2000005];
void init_perm() {
  perm[0] = 1;
  REP(i, 2000004) { perm[i] = perm[i - 1] * i % mod; }
}

int nCk(int x, int y) {
  if (y > x) {
    return 0;
  }
  if (x < 0) {
    return 0;
  }
  return perm[x] * modpow(perm[x - y], mod - 2) % mod *
         modpow(perm[y], mod - 2) % mod;
}

struct Dot {
  double x;
  double y;
  Dot(double _x = 0.0, double _y = 0.0) {
    x = _x;
    y = _y;
  }
};

struct Dot_i {
  int x;
  int y;
  Dot_i(int _x = 0, int _y = 0) {
    x = _x;
    y = _y;
  }
};

double kyori(pair<int, int> f, pair<int, int> s) {
  double ans = 0;
  double t = fabs(f.first - s.first);
  double y = fabs(f.second - s.second);
  ans = sqrt(t * t + y * y);
  return ans;
}

double kyori(Dot f, Dot s) {
  double ans = 0;
  double t = fabs(f.x - s.x);
  double y = fabs(f.y - s.y);
  ans = sqrt(t * t + y * y);
  return ans;
}

inline string stringmax(string &x, string &y) {
  if (x.size() > y.size()) {
    return x;
  }
  if (x.size() < y.size()) {
    return y;
  }
  rep(i, x.size()) {
    if (x[i] > y[i]) {
      return x;
    }
    if (x[i] < y[i]) {
      return y;
    }
  }
  return x;
}

// vector<int>  RollingHash(string &s, string& t){
//     vector<int> ans;
//     __int128 h=0,hamod=0,ki=0,kim=0,hikaku=0,one=0;
//     one=1;
//     ki=1000000007ll;
//     hamod=(one<<61)-1;
//     kim=1;
//     rep(i,t.size()){
//         hikaku*=ki;
//         h*=ki;
//         kim*=ki;
//         hikaku+=t[i];
//         h+=s[i];
//         hikaku%=hamod;
//         h%=hamod;
//         kim%=hamod;
//     }
//     rep(i,(int)s.size()-(int)t.size()+1){
//         if (h==hikaku) {
//             ans.emplace_back(i);
//         }
//         h*=ki;
//         h%=hamod;
//         h+=s[i+(int)t.size()];
//         h%=hamod;
//         h+=hamod;
//         h-=s[i]*kim%hamod;
//         h%=hamod;
//     }
//     return ans;
// }
struct edge {
  int to;
  int length;
  edge(int _to, int _length) {
    to = _to;
    length = _length;
  }
};
vector<int> djkstra(vector<vector<edge>> &road, int start) {
  vector<int> kyo(road.size(), inf);
  smallpriority_queue(P) q;
  q.push({0, start});
  kyo[start] = 0;
  while (q.size()) {
    int x = q.top().second;
    itn now = q.top().first;
    q.pop();
    if (kyo[x] < now) {
      continue;
    }
    for (auto &i : road[x]) {
      if (kyo[i.to] > now + i.length) {
        kyo[i.to] = now + i.length;
        q.push({kyo[i.to], i.to});
      }
    }
  }
  return kyo;
}

template <class T> void change_to_unique(vector<T> &v) {
  std::sort(ALL(v));
  auto k = unique(ALL(v));
  if (k != v.end())
    v.erase(k, v.end());
}

double kodo_dosuu(double r) { return 180.0 * r / (double)PI; }

double dosuu_kodo(double r) { return r * (double)PI / 180.0; }

double kakudo(Dot a, Dot b1, Dot b2) {
  double size1 = kyori(a, b1), size2 = kyori(a, b2);
  double nai = (b1.x - a.x) * (b2.x - a.x) + (b1.y - a.y) * (b2.y - a.y);
  nai /= size1 * size2;
  return kodo_dosuu(acos(nai));
}

struct fraction {
  int shi;
  int bo;
  fraction(int bunshi, int bunbo) : bo(bunbo), shi(bunshi) {
    if (bunbo && bunshi) {
      int g = gcd(bo, shi);
      bo /= g;
      shi /= g;
    }
  };
  explicit inline operator double() const { return (double)shi / (double)bo; }
  explicit inline operator long double() const {
    return (long double)shi / (long double)bo;
  }
};
bool operator<(const fraction &b1, const fraction &b2) {
  return b1.shi * b2.bo < b2.shi * b1.bo;
}
bool operator>(const fraction &b1, const fraction &b2) {
  return b1.shi * b2.bo > b2.shi * b1.bo;
}

template <class T> void vecout(vector<T> &v) {
  if (v.size()) {
    cout << v[0];
  }
  REP(i, (int)v.size() - 1) { cout << ' ' << v[i]; }
  cout << endl;
}

#define endl "\n" // interactive の時に注意！！！
#define Endl "\n" // interactive の時に注意！！！
#define cinn cin
#define printd cout << fixed << setprecision(10)
#define rrep(i, f, s) for (int i = f; i < s; i++)
#define RREP(i, f, s) for (int i = f; i <= s; i++)
#define REV_RREP(i, f, s) for (int i = s; i >= f; i--)
#define hen_rep(i, l, step) for (int i = 0; i < l; i += step)
#define HEN_REP(i, l, step) for (int i = 1; i <= l; i += step)

int n, m, x, y, ans;
vector<int> road[100004], topo;
int dp[100004];
int dfs(int x) {
  if (dp[x] != -1) {
    return dp[x];
  }
  int ka = 0;
  for (auto &i : road[x]) {
    chmax(ka, dfs(i) + 1);
  }
  return ka;
}
signed main() {
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cin >> n >> m;
  rep(i, m) {
    cin >> x >> y;
    road[x].emplace_back(y);
  }
  REP(i, n) dp[i] = -1;
  REP(i, n) chmax(ans, dfs(i));
  cout << ans << Endl;
}


Fixed submission:
/*
 * じょえチャンネル
 * 高評価・チャンネル登録よろしくおねがいします！
 * https://www.youtube.com/channel/UCRXsI3FL_kvaVL9zoolBfbQ
 */

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

// here!!!
//  define int long long !!!!!

#define int long long

// define int long long !!!!!

#define mod 1000000007ll
// constexpr int mod = 998244353ll;

constexpr double PI = 3.141592653589793238462643383279;
// constexpr double eps = DBL_EPSILON;

typedef long long ll;

#ifdef int
#define inf (int)(3e18)
#else
#define inf (int)(5e8)
#endif

#define intt long long
#define itn long long
#define innt long long
#define P pair<long long, long long>

#define rep(i, n) for (int i = 0; i < n; i++)
#define REP(i, n) for (int i = 1; i <= n; i++)
#define rev_rep(i, n) for (int i = n - 1; i >= 0; i--)
#define REV_REP(i, n) for (int i = n; i >= 1; i--)

#define ALL(v) v.begin(), v.end()

#define smallpriority_queue(T) priority_queue<T, vector<T>, greater<T>>

using namespace std;

// Library
// モッドパウ
inline int modpow(int x, int y, int m = mod) {
  int res = 1;
  while (y) {
    if (y & 1) {
      res *= x;
      res %= m;
    }
    x = x * x % m;
    y /= 2;
  }
  return res;
}

int mypow(int x, int y) {
  int res = 1;
  while (y) {
    if (y % 2) {
      res *= x;
    }
    x = x * x;
    y /= 2;
  }
  return res;
}
// is the number (x) a prime number?
bool prime(int x) {
  if (!x || x == 1) {
    return false;
  }
  for (int i = 2; i * i <= x; i++) {
    if (!(x % i)) {
      return false;
    }
  }
  return true;
}

// saidai-kouyakusuu
inline int gcd(int x, int y) {
  if (!y) {
    return x;
  }
  return gcd(y, x % y);
}

// number of keta
int keta(int x) {
  int ans = 0;
  while (x) {
    x /= 10;
    ans++;
  }
  return ans;
}

// number of 2shinsuu keta
int bitketa(int x) {
  int ans = 0;
  while (x) {
    x >>= 1;
    ans++;
  }
  return ans;
}

// sum of keta
int ketasum(int x) {
  int ans = 0;
  while (x) {
    ans += x % 10;
    x /= 10;
  }
  return ans;
}

inline int lcm(int x, int y) {
  int ans = x / gcd(x, y) * y;
  return ans;
}
int twobeki(int x) {
  int ans = 0;
  while (1) {
    if (!(x & 1)) {
      ans++;
      x >>= 1;
    } else {
      break;
    }
  }
  return ans;
}

template <class T, class U> inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
template <class T, class U> inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return 1;
  }
  return 0;
}
void Yes() { cout << "Yes" << endl; }
void No() { cout << "No" << endl; }
void YES() { cout << "YES" << endl; }
void NO() { cout << "NO" << endl; }

#define fin(i) scanf("%lld", &i)
#define fout(i) printf("%lld", i)
#define fendl printf("\n")

int kai(int x, int y) {
  int res = 1;
  for (int i = x - y + 1; i <= x; i++) {
    res *= i;
    res %= mod;
  }
  return res;
}

int comb(int x, int y) {
  if (y > x)
    return 0;
  //    cout<<kai(x, y)<<' '<<modpow(kai(y, y), mod - 2)<<endl;
  return kai(x, y) * modpow(kai(y, y), mod - 2) % mod;
}
// Library-End

// 入出力高速化時にはoff！！！！
// #define vecin(v) for(int i=0;i<v.size();i++)scanf("%lld",&v[i]);
// #define vecout(v) {if(v.size())printf("%lld",v[0]);for(int
// i=1;i<(int)v.size();i++)printf(" %lld",v[i]);printf("\n");}

template <class T> class SegTree {
  int n;
  vector<T> node;
  T def;
  function<T(T, T)> operation;
  function<T(T, T)> update;

public:
  SegTree(unsigned int _n, T _def, function<T(T, T)> _operation,
          function<T(T, T)> _update)
      : def(_def), operation(_operation), update(_update) {
    n = 1;
    while (n < _n) {
      n *= 2;
    }
    node = vector<T>(n * 2, def);
  }
  SegTree(vector<int> &initvec, function<T(T, T)> _operation,
          function<T(T, T)> _update)
      : operation(_operation), update(_update) {
    n = 1;
    while (n < initvec.size()) {
      n *= 2;
    }
    node = vector<T>(n * 2, def);
    for (int i = n; i < n + initvec.size(); i++) {
      node[i] = initvec[i - n];
    }
    for (int i = n - 1; i >= 1; i--) {
      node[i] = operation(node[i * 2], node[i * 2 + 1]);
    }
  }
  void change(int i, T x) {
    i += n;
    node[i] = update(node[i], x);
    while (i >= 1) {
      i >>= 1;
      node[i] = operation(node[i * 2], node[i * 2 + 1]);
    }
  }
  T query(int l, int r) {
    l += n;
    r += n;
    T rx = def, lx = def;
    while (l < r) {
      if (l & 1) {
        lx = operation(lx, node[l]);
        l++;
      }
      if (r & 1) {
        r--;
        rx = operation(node[r], rx);
      }
      l >>= 1;
      r >>= 1;
    }
    return operation(lx, rx);
  }
  T operator[](const int &x) { return node[x + n]; }
  void fill(T x) { std::fill(ALL(node), x); }
  void print() {
    rep(i, n) std::cout << operator[](i) << " ";
    std::cout << std::endl;
  }
  T queryForALL() { return node[1]; }
};

#define R_MIN ([](long long a, long long b) { return min(a, b); })
#define R_MAX ([](long long a, long long b) { return max(a, b); })
#define R_SUM ([](long long a, long long b) { return a + b; })

#define NORMAL_UPDATE ([](long long a, long long b) { return b; })
#define ADD_UPDATE ([](long long a, long long b) { return a + b; })
#define MINUS_UPDATE ([](long long a, long long b) { return a - b; }

class Union_Find {
  vector<int> par;
  vector<int> ookisa;

public:
  Union_Find(int size) {
    par = vector<int>(size);
    ookisa = vector<int>(size);
    for (int i = 0; i < size; i++) {
      par[i] = i;
      ookisa[i] = 1;
    }
  }

  int find(int x) {
    if (par[x] == x) {
      return x;
    }
    return par[x] = find(par[x]);
  }

  void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if (x == y) {
      return;
    }
    if (ookisa[x] < ookisa[y]) {
      par[x] = y;
      ookisa[y] += ookisa[x];
      ookisa[x] = 0;
    } else {
      par[y] = x;
      ookisa[x] += ookisa[y];
      ookisa[y] = 0;
    }
  }
  int size(int i) {
    i = find(i);
    return ookisa[i];
  }
  bool same(int x, int y) { return find(x) == find(y); }
};

class BIT {
  vector<int> data;
  int size = 0;

public:
  BIT(int _size) {
    data = vector<int>(_size + 1);
    size = _size;
  }
  void add(int i, int x) {
    while (i <= size) {
      data[i] += x;
      i += i & -i;
    }
  }
  int sum(int i) {
    assert(i <= size);
    int s = 0;
    while (i > 0) {
      s += data[i];
      i -= i & -i;
    }
    return s;
  }
  int lower_bound(int x) {
    if (x <= 0) {
      return 0;
    } else {
      int i = 0;
      int r = 1;
      while (r < size)
        r = r << 1;
      for (int len = r; len > 0; len = len >> 1) {
        if (i + len < size && data[i + len] < x) {
          x -= data[i + len];
          i += len;
        }
      }
      return i + 1;
    }
  }
};

// Union-Find-End

int perm[2000005];
void init_perm() {
  perm[0] = 1;
  REP(i, 2000004) { perm[i] = perm[i - 1] * i % mod; }
}

int nCk(int x, int y) {
  if (y > x) {
    return 0;
  }
  if (x < 0) {
    return 0;
  }
  return perm[x] * modpow(perm[x - y], mod - 2) % mod *
         modpow(perm[y], mod - 2) % mod;
}

struct Dot {
  double x;
  double y;
  Dot(double _x = 0.0, double _y = 0.0) {
    x = _x;
    y = _y;
  }
};

struct Dot_i {
  int x;
  int y;
  Dot_i(int _x = 0, int _y = 0) {
    x = _x;
    y = _y;
  }
};

double kyori(pair<int, int> f, pair<int, int> s) {
  double ans = 0;
  double t = fabs(f.first - s.first);
  double y = fabs(f.second - s.second);
  ans = sqrt(t * t + y * y);
  return ans;
}

double kyori(Dot f, Dot s) {
  double ans = 0;
  double t = fabs(f.x - s.x);
  double y = fabs(f.y - s.y);
  ans = sqrt(t * t + y * y);
  return ans;
}

inline string stringmax(string &x, string &y) {
  if (x.size() > y.size()) {
    return x;
  }
  if (x.size() < y.size()) {
    return y;
  }
  rep(i, x.size()) {
    if (x[i] > y[i]) {
      return x;
    }
    if (x[i] < y[i]) {
      return y;
    }
  }
  return x;
}

// vector<int>  RollingHash(string &s, string& t){
//     vector<int> ans;
//     __int128 h=0,hamod=0,ki=0,kim=0,hikaku=0,one=0;
//     one=1;
//     ki=1000000007ll;
//     hamod=(one<<61)-1;
//     kim=1;
//     rep(i,t.size()){
//         hikaku*=ki;
//         h*=ki;
//         kim*=ki;
//         hikaku+=t[i];
//         h+=s[i];
//         hikaku%=hamod;
//         h%=hamod;
//         kim%=hamod;
//     }
//     rep(i,(int)s.size()-(int)t.size()+1){
//         if (h==hikaku) {
//             ans.emplace_back(i);
//         }
//         h*=ki;
//         h%=hamod;
//         h+=s[i+(int)t.size()];
//         h%=hamod;
//         h+=hamod;
//         h-=s[i]*kim%hamod;
//         h%=hamod;
//     }
//     return ans;
// }
struct edge {
  int to;
  int length;
  edge(int _to, int _length) {
    to = _to;
    length = _length;
  }
};
vector<int> djkstra(vector<vector<edge>> &road, int start) {
  vector<int> kyo(road.size(), inf);
  smallpriority_queue(P) q;
  q.push({0, start});
  kyo[start] = 0;
  while (q.size()) {
    int x = q.top().second;
    itn now = q.top().first;
    q.pop();
    if (kyo[x] < now) {
      continue;
    }
    for (auto &i : road[x]) {
      if (kyo[i.to] > now + i.length) {
        kyo[i.to] = now + i.length;
        q.push({kyo[i.to], i.to});
      }
    }
  }
  return kyo;
}

template <class T> void change_to_unique(vector<T> &v) {
  std::sort(ALL(v));
  auto k = unique(ALL(v));
  if (k != v.end())
    v.erase(k, v.end());
}

double kodo_dosuu(double r) { return 180.0 * r / (double)PI; }

double dosuu_kodo(double r) { return r * (double)PI / 180.0; }

double kakudo(Dot a, Dot b1, Dot b2) {
  double size1 = kyori(a, b1), size2 = kyori(a, b2);
  double nai = (b1.x - a.x) * (b2.x - a.x) + (b1.y - a.y) * (b2.y - a.y);
  nai /= size1 * size2;
  return kodo_dosuu(acos(nai));
}

struct fraction {
  int shi;
  int bo;
  fraction(int bunshi, int bunbo) : bo(bunbo), shi(bunshi) {
    if (bunbo && bunshi) {
      int g = gcd(bo, shi);
      bo /= g;
      shi /= g;
    }
  };
  explicit inline operator double() const { return (double)shi / (double)bo; }
  explicit inline operator long double() const {
    return (long double)shi / (long double)bo;
  }
};
bool operator<(const fraction &b1, const fraction &b2) {
  return b1.shi * b2.bo < b2.shi * b1.bo;
}
bool operator>(const fraction &b1, const fraction &b2) {
  return b1.shi * b2.bo > b2.shi * b1.bo;
}

template <class T> void vecout(vector<T> &v) {
  if (v.size()) {
    cout << v[0];
  }
  REP(i, (int)v.size() - 1) { cout << ' ' << v[i]; }
  cout << endl;
}

#define endl "\n" // interactive の時に注意！！！
#define Endl "\n" // interactive の時に注意！！！
#define cinn cin
#define printd cout << fixed << setprecision(10)
#define rrep(i, f, s) for (int i = f; i < s; i++)
#define RREP(i, f, s) for (int i = f; i <= s; i++)
#define REV_RREP(i, f, s) for (int i = s; i >= f; i--)
#define hen_rep(i, l, step) for (int i = 0; i < l; i += step)
#define HEN_REP(i, l, step) for (int i = 1; i <= l; i += step)

int n, m, x, y, ans;
vector<int> road[100004], topo;
int dp[100004];
int dfs(int x) {
  if (dp[x] != -1) {
    return dp[x];
  }
  int ka = 0;
  for (auto &i : road[x]) {
    chmax(ka, dfs(i) + 1);
  }
  return dp[x] = ka;
}
signed main() {
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  cin >> n >> m;
  rep(i, m) {
    cin >> x >> y;
    road[x].emplace_back(y);
  }
  REP(i, n) dp[i] = -1;
  REP(i, n) chmax(ans, dfs(i));
  cout << ans << Endl;
}