// Slower C++ solution:


/**

    And in the end, the love you take is equal to the love you make.



        I LOVE YOU LAURA.

**/



#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

#include <ext/rope>



#define _CRT_SECURE_NO_DEPRECATE

#define _CRT_DISABLE_PERFCRIT_LOCKS



#define findOrder find_by_order

#define orderKey order_of_key



#define isOn(mask, i) (mask & (1 << i))

#define turn(mask, i) (mask |= (1 << i))

#define switch(mask, i) (mask &= ~(1 << i))

#define change(mask, i) (mask ^= (1 << i))

#define lowbit(mask) (mask & (-mask))

#define turnAll(mask, n) (mask = (1 << n) - 1)

#define switchLastBit(mask) (mask & (mask - 1))

#define turnLastZero(mask) (mask | (mask + 1))

#define modulo(x, mod) ((x) & (mod - 1))

#define log2(x) (31 - __builtin_clz(x))



#define mkp make_pair

#define pb push_back

#define pf push_front

#define eb emplace_back

#define ef emplace_front

#define ins insert

#define rsz resize

#define f first

#define s second

#define lb lower_bound

#define ub upper_bound



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

#define all(x) begin(x), end(x)

#define rall(x) rbegin(x), rend(x)



#define debug(x) (cerr << x << " ")



using namespace std;

using namespace __gnu_pbds;

using namespace __gnu_cxx;



typedef double db;

typedef long long ll;

typedef long double ld;

typedef unsigned long long ull;



typedef pair<int, int> ii;

typedef pair<ll,ll> pll;

typedef pair<ld,ld> pdb;

typedef pair<int, ii> tri;

typedef complex<ld> cd;



typedef vector<int> vi;

typedef vector<ii> vii;

typedef vector<ld> vdb;

typedef vector<cd> vcd;

typedef vector<ll> vll;

typedef vector<string> vs;

typedef vector<vll> matrix;



typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> indexed_tree;



const ll mod = 1e9+7;

const int maxn = 1e5+2;

const int pi = 2 * acos(0.0);

const db eps = 1e-9;



ll n, m;

vll graph[maxn];

ll dp[maxn];



ll dfs (int node){

    ll &ans = dp[node];



    if (ans != -1) return ans;



    for (auto v: graph[node]){

        ans = max(ans, dfs(v) + 1);

    }

    return ans;

}



int main()

{

    ios_base::sync_with_stdio(0);

    cin.tie(0); cout.tie(0);



    memset(dp, -1, sizeof dp);



    cin >> n >> m;



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

        int a, b; cin >> a >> b;



        graph[a].pb(b);

    }



    ll ans = 0;

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

        ans = max(ans, dfs(i) + 1);

    }



    cout << ans << "\n";



    return 0;

}


// Faster C++ solution for the same problem (measured speedup: 1.2371982070274345x):
#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;















}