//Author: Fuadul Hasan(fuadul202@gmail.com)
//BSMRSTU,Gopalganj
#include<bits/stdc++.h>
using namespace std;
//int input........
template <typename T> inline void readline(T &n) {n = 0; int f = 1; register int ch = getchar();
 for (; !isdigit(ch); ch = getchar()) if (ch == '-') f = -1;for (; isdigit(ch); 
ch = getchar()) n = (n << 3) + (n << 1) + ch - '0';n = n * f;}
template <typename T, typename TT> inline void readline(T &n, TT &m) { readline(n); readline(m); }
template <typename T, typename TT, typename TTT> inline void readline(T &n, TT &m, TTT &l) { readline(n, m); readline(l);}
// long long input........
template <typename T> inline void readlinel(T &n) {n = 0; long long f = 1; register long long ch = getchar();
 for (; !isdigit(ch); ch = getchar()) if (ch == '-') f = -1;for (; isdigit(ch); 
ch = getchar()) n = (n << 3) + (n << 1) + ch - '0';n = n * f;}
template <typename T, typename TT> inline void readlinel(T &n, TT &m) { readlinel(n); readlinel(m); }
template <typename T, typename TT, typename TTT> inline void readlinel(T &n, TT &m, TTT &l) { readlinel(n, m); readlinel(l);}
//debug..........
#define error(args...) {vector<string>_v=split(#args,',');err(_v.begin(),args);cout<<endl;}
vector<string> split(const string &s, char c) {vector<string>v; stringstream ss(s); string x;
while (getline(ss, x, c))v.emplace_back(x); return move(v);} void err(vector<string>::iterator it) {}
template<typename T, typename... Args>void err(vector<string>::iterator it, T a, Args...args) {
cout << it->substr((*it)[0] == ' ', it->length()) << " = " << a << " "; err(++it, args...);}
//............ignore it..................//
int dx[8]= {1,0,-1,0,-1,-1,1,1};int dy[8]= {0,1,0,-1,-1,1,-1,1}; int tc = 1;
#define pb  push_back
#define mp make_pair
#define ll  long long int
#define endl  '\n';
#define happy cin.tie(0);
#define coding ios::sync_with_stdio(false);
#define Test printf("Case %d: ",tc++);
#define YES printf("YES\n");
#define NO printf("NO\n");
#define Yes printf("Yes\n");
#define No printf("No\n");
void print(int n){printf("%d ", n);}
void print(int n,int m){printf("%d ",n);printf("%d ",m);}
void print(int n,int m,int k){printf("%d ",n);printf("%d ",m);printf("%d ",k);}
void print(ll n){printf("%I64d ", n);}
void print(ll n,ll m){printf("%I64d ",n);printf("%I64d ",m);}
void print(ll n,ll m,ll k){printf("%I64d ",n);printf("%I64d ",m);printf("%I64d ",k);}
#define   pr 	pair<int, int> 
#define   vpr 	vector<pr> 
#define   vi   	std::vector<int>
#define   vll  	std::vector<ll>
#define   map   std::map<string, int>
#define   all(n) n.begin(),n.end()
const int mod           = 1e9 + 7;
const int Inf           = (int)2e9 + 5;
const ll  Lnf           = (ll)2e18 + 5;
const int N             = 4e5 + 5;
const int N1            = 1e5 + 5;
std::vector<int> eg[100001];
int vis[100001],dp[100001];

int dfs(int node){
	vis[node] = 1;
	for(int child: eg[node]){
		if(!vis[child]){
			dfs(child);
		}
		dp[node] = max(dp[node],1+dp[child]);
	}
}

int solve() 
{
	//happy coding

	int n,e;
	cin>>n>>e;
	for(int i=0;i<e;i++){
		int u,v;
		cin>>u>>v;
		eg[u].pb(v);
	}
	
	for(int i=1;i<=n;i++){
		if(!vis[i]){
			dfs(i);
		}
	}

	/*for(int i=1;i<=n;i++){
		cout<<dp[i]<<" ";
	}cout<<endl;*/

	int ans = 0;
	for(int i=1;i<=n;i++){
		ans = max(ans,dp[i]);
	}


	cout<<ans<<endl;
	return 0;
	//error();
}
int main(){
	int test = 1;
	//readline(test);//(void)getchar();
	while (test--)solve();return 0;
}

Measured memory_bytes: 8576