/******************************
 * 
 * created by: Shivam prajapati
 * 
 * *****************************/
#include<bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 

using namespace __gnu_pbds; 
using namespace std;

typedef long long int ll;
typedef long long unsigned int llu;
const int INF = 0x3f3f3f3f;

#define ff first
#define ss second
#define ins insert
#define tc ll t; cin>>t; while(t--)

#define  sync ios_base::sync_with_stdio(false);  cin.tie(NULL);
#define pb push_back
#define pf pop_front
#define mp make_pair

#define mpi unordered_map<ll, ll>
#define pr pair<ll, ll>
#define vi vector<ll>
#define vc vector<char>

#define mpc unordered_map<char, ll>
#define it_loop(it,m) for(auto it = m.begin(); it!=m.end(); it++)
#define rep(i,n) for(ll i=0;i<n;i++)
#define repx(i,begin, end) for(ll i=begin; i<=end;i++)

#define endl '\n'
#define ordered_set tree<ll, null_type,less<ll>, rb_tree_tag,tree_order_statistics_node_update> 
#define mod 1e9+7
#define count_ones __builtin_popcountl              // #ones in binary rep.
#define what_is(x) cerr << #x << " is " << x << endl  //#x -> actual name of variable

#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
#define eb emplace_back                    // faster than push_back 
#define mt make_tuple                        //  for returning more than one value with tie(args ...) = tuple
#define fill(a,n,start) iota(a,a+n,start)    // fill array in increasing order

#define shuffle(a,n) shuffle(a,a+n,default_random_engine(0))
#define print_arr(a,n) rep(i,n) cout<<a[i]<<endl
#define read_arr(a,n) rep(i,n) cin>>a[i]
#define ook order_of_key   // ook(n) It returns to the number of items that are strictly smaller than our item n

#define nds new_data_set    //pbds array
#define fbo find_by_order   // fbo(i), returns an iterator at the ith index

template<typename... T>
void read(T&... args){
  ((cin>>args), ...);
}

template<typename... T>
void write(T... args){
  ((cout<<args<<'\n'), ...);
}

void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
	cerr << *it << " = " << a << endl;
	err(++it, args...);
}

#if 0
void add(int idx, int val,int n,int bit[]){
   for(;idx<n;idx = idx|(idx+1))  bit[idx]+=val;
}
int sum(int r,int bit[]){
	int  res = 0;	
	for(;r>=0;r=(r&(r+1))-1) res+=bit[r];
	return res;
}
int sum_range(int l,int r,int bit[]){
	return sum(r,bit)-sum(l-1,bit);
}
void init(int n,int arr[],int bit[]){	
	for(int i=0;i<n;i++)  add(i,arr[i],n,bit);
}
#endif

/* gcd __gcd(a,b)
 * sparse_table: st[i][j] = f(st[i][j-1],st[i+(1<<(j-1))][j-1])
 * sparse_table: for range query in immutable array
 * modified queue: stack<pair<ll,ll>> s1,s2   s1->add,  s2->delete
 * long long binpow(long long a, long long b) {    long long res = 1;while (b > 0) {if (b & 1)res = res * a;a = a * a;b >>= 1;}return res;}
 * " \n"[0] is ' ' and " \n"[1] is '\n'
 * we can also use ignore keyword in tie to ignore a value ->  tie(b, ignore, a, ignore)
 * 0 based indexing in pbds ordered_set
 * add(l,x) & add(r+1,-x) for BIT range update point query
*/
int in_deg[100001]={0};
vi g[100001];
int dp[100001]={0};

void dfs(int v){
	
	for(int u:g[v]){
		dp[u]=max(dp[u],dp[v]+1);
		in_deg[u]--;
		if(in_deg[u]==0)
			dfs(u);
	}
	
	
}


void solve(){
	int n,m;
	cin>>n>>m;
	int u,v;
	
	rep(i,m){
		cin>>u>>v;
		g[u].eb(v);
		in_deg[v]++;
	}
	
	for(int i=1;i<=n;i++){ 
		if(in_deg[i]==0){
			g[0].eb(i);
			in_deg[i]++;
		}
	}	
	dfs(0);
	int mx=-1;
	for(int i=1;i<=n;i++){
		if(dp[i]>mx) mx = dp[i];
	}
	cout<<mx-1<<endl;
}

int main(){
  
  sync;
  //tc{ solve(); }
  solve();
}




Measured memory_bytes: 14480