// Slower C++ solution:
#include <bits/stdc++.h>



using namespace std;



#define el				'\n'

#define sz(v)			((int)((v).size()))

#define all(v)          ((v).begin()),((v).end())

#define clr(v, d)		memset(v, d, sizeof(v))



double const EPS = 1e-8, PI = acos(-1);

const int N = 1e5 + 9, M = 50 + 9, OO = (int) 1e9 + 1;

const long long MOD = 1e9 + 7;



typedef long long ll;



void OUTPUT() {

	cout << fixed << setprecision(12);

	ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);

}



void INPUT() {

#ifndef ONLINE_JUDGE

	freopen("input.txt", "r", stdin);

	//freopen("output.txt", "w", stdout);

#endif

	cout << fixed << setprecision(0);

//	ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);

}



int n, m, x, y, vis[N], ans;

vector<vector<int>> edges(N);



int DFS(int node) {

	if (vis[node] != -1)

		return vis[node];

	vis[node] = 0;

	for (auto child : edges[node]) {

		DFS(child);

		vis[node] = max(vis[node], vis[child] + 1);

	}

	return 0;

}



int main() {

//	INPUT();

	OUTPUT();

	clr(vis, -1);

	cin >> n >> m;

	for (int i = 0; i < m; ++i) {

		cin >> x >> y;

		edges[x].push_back(y);

	}

	for (int j = 1; j <= n; j++) {

		DFS(j);

	}

	for (int i = 1; i <= n; i++) {

		ans = max(ans, vis[i]);

	}

	cout << ans;



	return 0;



}


// Faster C++ solution for the same problem (measured speedup: 1.1197047196709418x):
#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

typedef unsigned long long ull;

typedef pair<ll,ll>pll;

typedef pair<ll,pair<ll,ll>>plll;

#define fastread() (ios_base:: sync_with_stdio(false),cin.tie(NULL));

#define vll(v) v.begin(),v.end()

#define all(x) x.rbegin(),x.rend()

#define min3(a, b, c)   min(a, min(b, c))

#define max3(a, b, c)   max(a, max(b, c))

#define F first

#define S second

#define in  freopen("input.txt", "r", stdin)

#define out  freopen("output.txt", "w", stdout)

#define minheap int,vector<int>,greater<int>

#define pb push_back

#define eb emplace_back

#define ischar(x)  (('a' <= x && x <= 'z') || ('A' <= x && x <= 'Z'))

#define isvowel(ch) ((ch=='a'||ch=='e'||ch=='i'||ch=='o'||ch=='u')||(ch=='A'|| ch=='E' || ch=='I'|| ch=='O'|| ch=='U'))

#define bug cout<<"BUG"<<endl;

const int Max = 1e5 + 10;

const int Mod = 1e9 + 7;

const double PI  =3.141592653589793238463;





ll n,m;



vector<ll>adj[Max+10];



ll indeg[Max+10];



ll dis[Max+10];







void input()

{

      ll i,a,b;





       for(i=1; i<=m; i++)

       {

           cin>>a>>b;



           adj[a].eb(b);



           indeg[b]++;

       }

}



ll topsort()

{

    ll u,mx=0;



       queue<ll>q;



       for(ll i=1; i<=n; i++)

       {

           if(!indeg[i])q.push(i);

       }



         while(!q.empty())

         {

              u=q.front();



              q.pop();



                for(auto v  : adj[u])

                {

                   dis[v]=max(dis[v],dis[u]+1);



                   mx=max(dis[v],mx);



                   indeg[v]--;



                   if(!indeg[v])

                    q.push(v);

                }

         }



         return mx;

}

int main()

{



   fastread();



   ll i,j,p,a,sum=0,k,t,b,c,d,cnt=0,q,l,r;





     string str;



   bool flag=true;



     cin>>n>>m;



     input();





      cout<<topsort()<<endl;















}