#ifndef LOCAL
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target ("avx")
#endif

#include <vector>
#include <set>
#include <map>
#include <tuple>
#include <queue>
#include <stack>
#include <bitset>
#include <functional>
#include <algorithm>
#include <utility>
#include <numeric>
#include <iostream>
#include <cmath>
#include <cassert>
#include <cstring>

using namespace std;

using ll = long long;
using VI = vector<int>;
using VVI = vector<vector<int>>;
using VLL = vector<ll>;
using VVLL = vector<vector<ll>>;
using VB = vector<bool>;
using PII = pair<int, int>;
using PLL = pair<ll, ll>;
template <typename T>
using minheap = priority_queue<T, vector<T>, greater<T>>;
const int INF = 1000000007;
const ll INF_LL = 1'000'000'000'000'000'007;

#define __overload3(_1, _2, _3, name,...) name
#define rep(...) __overload3(__VA_ARGS__, repFromUntil, repUntil, repeat)(__VA_ARGS__)
#define repeat(times) repFromUntil(_name##__LINE__, 0, times)
#define repUntil(name, times) repFromUntil(name, 0, times)
#define repFromUntil(name, from, until) for (int name = from, name##__until = (until); name < name##__until; name++)
#define repr(...) __overload3(__VA_ARGS__, reprFromUntil, reprUntil, repeat)(__VA_ARGS__)
#define reprUntil(name, times) reprFromUntil(name, 0, times)
#define reprFromUntil(name, from, until) for (int name = (until) - 1, name##__from = (from); name >= name##__from; name--)

#define EXIT(out) do { OUT(out); exit(0); } while (0)

#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define fs first
#define sn second

#ifdef LOCAL
#define debug(v) do {debugos << "L" << __LINE__ << " " << #v << " > ";debugos<<(v)<<newl;} while (0)
#define debugv(v) do {debugos << "L" << __LINE__ << " " << #v << " > ";for(auto e:(v)){debugos<<e<<" ";}debugos<<newl;} while (0)
#define debuga(m,w) do {debugos << "L" << __LINE__ << " " << #m << " > ";for(int x=0;x<(w);x++){debugos<<(m)[x]<<" ";}debugos<<newl;} while (0)
#define debugaa(m,h,w) do {debugos << "L" << __LINE__ << " " << #m << " > \n";for(int y=0;y<(h);y++){for(int x=0;x<(w);x++){debugos<<(m)[y][x]<<" ";}debugos<<newl;}} while (0)
#else
#define debug(v) do {v;} while (0)
#define debugv(v) do; while (0)
#define debuga(m,w) do; while (0)
#define debugaa(m,h,w) do; while (0)
#endif

#define newl "\n"
constexpr int dr[] = {1,-1,0,0};  // LRUD
constexpr int dc[] = {0,0,1,-1};

bool inside(int r, int c, int H, int W) {
  return 0 <= r and r < H and 0 <= c and c < W;
}

template <typename T, typename U> bool chmin(T& var, U x) {
  if (var > x) {
    var = x;
    return true;
  } else return false;
}

template <typename T, typename U> bool chmax(T& var, U x) {
  if (var < x) {
    var = x;
    return true;
  } else return false;
}

template <typename T> int sgn(T val) { return (T(0) < val) - (val < T(0)); }

ll power(ll e, int t, ll mod = INF_LL) {
  ll res = 1;
  while (t) {
    if (t&1) res = (res * e) % mod;
    t >>= 1;
    e = (e * e) % mod;
  }
  return res;
}

template <typename T> T divceil(T m, T d) {
  assert(m >= 0 and d > 0);
  return (m+d-1)/d;
}

template<typename T>
vector<T> make_v(size_t a, T b) { return vector<T>(a, b); }

template<typename... Ts>
auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v(ts...))>(a, make_v(ts...));
}

string operator*(const string& s, int times) {
  string res = "";
  rep(times) res += s;
  return res;
}

struct Scanner {
public:

  explicit Scanner(FILE *fp) : fp(fp) {}

  template< typename T, typename... E >
  void read(T &t, E &... e) {
    read_single(t);
    read(e...);
  }

private:
  static constexpr size_t line_size = 1 << 16;
  static constexpr size_t int_digits = 20;
  char line[line_size + 1] = {};
  FILE *fp = nullptr;
  char *st = line;
  char *ed = line;

  void read() {}

  static inline bool is_space(char c) {
    return c <= ' ';
  }

  void reread() {
    ptrdiff_t len = ed - st;
    memmove(line, st, len);
    char *tmp = line + len;
    ed = tmp + fread(tmp, 1, line_size - len, fp);
    *ed = 0;
    st = line;
  }

  void skip_space() {
    while(true) {
      if(st == ed) reread();
      while(*st && is_space(*st)) ++st;
      if(st != ed) return;
    }
  }

  template< typename T, enable_if_t< is_integral< T >::value, int > = 0 >
  void read_single(T &s) {
    skip_space();
    if(st + int_digits >= ed) reread();
    bool neg = false;
    if(is_signed< T >::value && *st == '-') {
      neg = true;
      ++st;
    }
    typename make_unsigned< T >::type y = *st++ - '0';
    while(*st >= '0') {
      y = 10 * y + *st++ - '0';
    }
    s = (neg ? -y : y);
  }

  template< typename T, enable_if_t< is_same< T, string >::value, int > = 0 >
  void read_single(T &s) {
    s = "";
    skip_space();
    while(true) {
      char *base = st;
      while(*st && !is_space(*st)) ++st;
      s += string(base, st);
      if(st != ed) return;
      reread();
    }
  }

  template< typename T >
  void read_single(vector< T > &s) {
    for(auto &d : s) read(d);
  }
} IN(stdin);

class MyPrinter {
public:
  template <typename T>
  void output_integer(T var) {
    if (var == 0) { putchar('0'); return; }
    if (var < 0) putchar('-'), var = -var;
    char stack[32]; int stack_p = 0;
    while (var) stack[stack_p++] = '0' + (var % 10), var /= 10;
    while (stack_p) putchar(stack[--stack_p]);
  }
  MyPrinter& operator<<(char c) { putchar(c); return *this; }
  MyPrinter& operator<<(double x) { printf("%.10f\n", x); return *this; }
  template <typename T> MyPrinter& operator<<(T var) { output_integer<T>(var); return *this; }
  MyPrinter& operator<<(char* str_p) { while (*str_p) putchar(*(str_p++)); return *this; }
  MyPrinter& operator<<(const char* str_p) { while (*str_p) putchar(*(str_p++)); return *this; }
  MyPrinter& operator<<(const string& str) {
    const char* p = str.c_str();
    const char* l = p + str.size();
    while (p < l) putchar(*p++);
    return *this;
  }
  template <typename T>
  void operator()(T x) { *this << x << newl; }
  template <typename T, typename... Ts>
  void operator()(T x, Ts ...xs) { *this << x << " "; this->operator()(xs...); }
  template <typename Iter>
  void iter(Iter s, Iter t) {
    if (s == t) *this << "\n";
    else {
      for (; s != t; s++) {
        *this << *s << " \n"[next(s, 1) == t];
      }
    }
  }
  template <typename Range>
  void range(const Range& r) { iter(begin(r), end(r)); }
} OUT;

class DebugPrint {
public:
  template <typename T>
  DebugPrint& operator <<(const T& v) {
#ifdef LOCAL
    cerr << v;
#endif
    return *this;
  }
} debugos;

template <typename OutStream, typename T, typename U>
OutStream& operator<<(OutStream& out, const pair<T, U>& var) {
  return out << var.first << " " << var.second;
}
template <typename OutStream, typename Tuple, size_t I, size_t N,
          enable_if_t<I == N>* = nullptr>
OutStream& tuple_impl(OutStream& out, const Tuple& var) {
  return out;
}
template <typename OutStream, typename Tuple, size_t I, size_t N,
          enable_if_t<I != N>* = nullptr>
OutStream& tuple_impl(OutStream& out, const Tuple& var) {
  out << get<I>(var) << " ";
  return tuple_impl<OutStream, Tuple, I+1, N>(out, var);
}
template <typename OutStream, typename... Ts>
OutStream& operator<<(OutStream& out, const tuple<Ts...>& var) {
  return tuple_impl<OutStream, tuple<Ts...>, 0, sizeof...(Ts)>(out, var);
}
template <typename InStream, typename T, typename U>
InStream& operator>>(InStream& in, pair<T, U>& var) {
  return in >> var.first >> var.second;
}
template <typename InStream, typename Tuple, size_t I, size_t N,
          enable_if_t<I == N>* = nullptr>
InStream& tuple_impl(InStream& in, Tuple& var) {
  return in;
}
template <typename InStream, typename Tuple, size_t I, size_t N,
          enable_if_t<I != N>* = nullptr>
InStream& tuple_impl(InStream& in, Tuple& var) {
  in >> get<I>(var);
  return tuple_impl<InStream, Tuple, I+1, N>(in, var);
}
template <typename InStream, typename... Ts>
InStream& operator>>(InStream& in, tuple<Ts...>& var) {
  return tuple_impl<InStream, tuple<Ts...>, 0, sizeof...(Ts)>(in, var);
}

#define MaxN 100005
#define MaxM 100005

int nxt[MaxM], head[MaxN], to[MaxM];
int memo[MaxN];
int n, m, x, y, res;

void dfs(int v) {
  if (~memo[v]) return;
  memo[v] = 0;
  for (int e = head[v]; ~e; e = nxt[e]) {
    dfs(to[e]);
    chmax(memo[v], memo[to[e]]+1);
  }
  chmax(res, memo[v]);
}

int main() {
  memset(memo, -1, sizeof(memo));
  memset(head, -1, sizeof(head));

  IN.read(n, m);
  rep(i, m) {
    IN.read(x, y); x--; y--;
    nxt[i] = head[x]; head[x] = i; to[i] = y;
  }

  rep(i, n) dfs(i);
  printf("%d\n", res);
}

Measured memory_bytes: 2688