#include<bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define EB emplace_back
#define MP make_pair
#define all(o) (o).begin(), (o).end()
#define mset(m,v) memset(m,v,sizeof(m))
#define fr(i,n) for(lli i=0;i<n;++i)
#define rep(i,a,b) for(lli i=a;i<=b;++i)
#define per(i,a,b) for(lli i=a;i>=b;i--)
#define remin(a,b) (a=min((a),(b)))
#define remax(a,b) (a=max((a),(b))) 
#define sz(x) (lli)(x).size()
#define endl '\n'
typedef long long lli;        typedef long double ld;
typedef pair<lli,lli> ii;     typedef vector<lli> vi;
typedef vector<ii> vii;       typedef vector<vi> graph;
long long MOD=1000000007;     long double EPS=1e-9;
#define debarr(a,n) cout<<#a<<" : ";for(int i=0;i<n;i++) cerr<<a[i]<<" "; cerr<<endl;
#define debmat(mat,row,col) cout<<#mat<<" :\n";for(int i=0;i<row;i++) {for(int j=0;j<col;j++) cerr<<mat[i][j]<<" ";cerr<<endl;}
#define pr(...) dbs(#__VA_ARGS__, __VA_ARGS__)
template <class S, class T>ostream& operator <<(ostream& os, const pair<S, T>& p) {return os << "(" << p.first << ", " << p.second << ")";}
template <class T>ostream& operator <<(ostream& os, const vector<T>& p) {os << "[ "; for (auto& it : p) os << it << " "; return os << "]";}
template <class T>ostream& operator <<(ostream& os, const unordered_set<T>& p) {os << "[ "; for (auto& it : p) os << it << " "; return os << "]";}
template <class S, class T>ostream& operator <<(ostream& os, const unordered_map<S, T>& p) {os << "[ "; for (auto& it : p) os << it << " "; return os << "]";}
template <class T>ostream& operator <<(ostream& os, const set<T>& p) {os << "[ "; for (auto& it : p) os << it << " "; return os << "]";}
template <class T>ostream& operator <<(ostream& os, const multiset<T>& p) {os << "[ "; for (auto& it : p) os << it << " "; return os << "]";}
template <class S, class T>ostream& operator <<(ostream& os, const map<S, T>& p) {os << "[ "; for (auto& it : p) os << it << " "; return os << "]";}
template <class T> void dbs(string str, T t) {cerr << str << " : " << t << "\n";}
template <class T, class... S> void dbs(string str, T t, S... s) {int idx = str.find(','); cerr << str.substr(0, idx) << " : " << t << ","; dbs(str.substr(idx + 1), s...);}
template <class T> void prc(T a, T b) {cerr << "["; for (T i = a; i != b; ++i) {if (i != a) cerr << ", "; cerr << *i;} cerr << "]\n";}
lli binpow(lli b,lli p,lli mod){lli ans=1;b%=mod;for(;p;p>>=1){if(p&1)ans=ans*b%mod;b=b*b%mod;}return ans;}

void pre(){

}

graph g;
int vis[100100];
vi topo;

void dfs(int nn){
	vis[nn]=1;
	for(auto v:g[nn]){
		if(!vis[v]){
			dfs(v);
		}
	}
	topo.EB(nn);
}

int pos[100100];
int dp[100100];


void solve(){
	int n,m;
	cin>>n>>m;
	g.resize(n+1);
	fr(i,m){
		int a,b;
		cin>>a>>b;
		g[a].EB(b);
	}	
	rep(i,1,n){
		if(!vis[i]){
			dfs(i);
		}
	}
	reverse(all(topo));
	fr(i,n){
		pos[topo[i]]=i;
	}
	fr(i,n){
		int v=topo[i];
		for(auto nn:g[v]){
			remax(dp[pos[nn]],dp[pos[v]]+1);
		}
	}
	int ans=0;
	fr(i,n){
		remax(ans,dp[i]);
	}
	cout<<ans<<endl;
}

signed main(){
	ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
	//freopen("in.txt","r",stdin);//freopen("out.txt","w",stdout);
	pre();lli t=1;//cin>>t;
	for(lli i=1;i<=t;i++){
		//cout<<"Case #"<<i<<": ";
		solve();
	}
}

Measured memory_bytes: 9844