#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif


using namespace std;

#define ld long double
#define ll long long int
#define vi vector<int>
#define vll vector<ll>
#define vvi vector < vi >
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vpll vector < pair<long long,long long> >
#define fill(a,val)  memset(a,val, sizeof (a))
#define sort_unique(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define mp make_pair
#define pb push_back
#define f first
#define se second
#define all(cont) cont.begin(), cont.end()
#define max3(a,b,c) max(a,max(b,c))
#define min3(a,b,c) min(a,min(b,c))
#define MAX   1e9
#define MIN  -1e9
typedef map<int,int> mpi;
typedef set<int> seti;
typedef multiset<int> mseti;
typedef priority_queue<int> pq;
typedef tuple< int , int, int > tup;
#define fio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define rep(i, n)    for(long long int i = 0; i < (n); ++i)
#define repA(i, a, n)  for(int i = a; i <= (n); ++i)
#define repD(i, a, n)  for(int i = a; i >= (n); --i)
#define trav(a, x) for(auto& a : x)
#define tr(c,i) for(typeof((c).begin() i = (c).begin(); i != (c).end(); i++)
#define repn(i, begin, end) for (__typeof(end) i = (begin) - ((begin) > (end)); i != (end) - ((begin) > (end)); i += 1 - 2 * ((begin) > (end)))
#define PI 3.14159265

#define mod 1000000007

struct node{
    ll val;
};

struct Interval {int start, end;};
bool compare(struct node n1, struct node n2) {
    return (n1.val < n2.val);
}

const ll INF = 1e18+3;
const ll inf = 1e9+5;
const ll maxn = 1e6+2;

vll adj[maxn];
map <int,int> ma ;
ll dp[maxn];

ll solve(ll u)
{
    if(dp[u]!=-1)
        return dp[u];
    if(ma[u])
    { dp[u]=0; return dp[u];}
    
    auto it = adj[u].begin();
    for(it=adj[u].begin();it!=adj[u].end();++it)
        dp[u]=max(solve(*it)+1,dp[u]);
    
    return dp[u];
    
}

int main()
{
    ll n , m ; cin>>n>>m; fill(dp,-1);
    rep(i,m){ ll u,v; cin>>u>>v ; adj[u].pb(v); }
    repA(i,1,n){ if((ll)adj[i].size()==0LL)++ma[i];}
    repA(i,1,n){ solve(i); }
    ll ans=-1;
    repA(i,1,n){ ans=max(ans,dp[i]);}
    cout<<ans<<endl;
    return 0;
}


Measured memory_bytes: 41344