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

#define f(i,s,v) for(int i=(int)s;i<(int)v;++i)

#define rf(i,s,v) for(int i=(int)s-1;i>(int)v;--i)

#define pb push_back

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

#define memset(c, value) memset(c, (int)value, sizeof(c))

using namespace std;

typedef long long ll;

const ll oo = 1e18;

const ll MOD =1e9+7;

const double pi = acos(-1);

void fast(){

    ios_base :: sync_with_stdio(0);

    cin.tie(0);

    cout.tie(0);

}

int n, m;

bool valid (int r,int c){

    return (r < n && r >= 0 && c >= 0 && c < m);

}



int row[] = {0 , 1 , 0, -1};

int column[] = {1 , 0, -1, 0};



vector<int> adj[100001];

int path[100001];

int vis[100001];



int dp(int u){

    if(path[u]!=-1)

        return path[u];



    vis[u] = true;



    int sz = adj[u].size();



    path[u] = 0;

    f(j, 0, sz){

        if(!vis[adj[u][j]])

            path[u] = max( path[u], 1 + dp(adj[u][j]));

    }

    vis[u] = false;



    return path[u];

}





int main()

{

    fast();



    int u, v;

    cin>>n>>m;

    f(i, 0, m){

        cin>>u>>v;

        adj[u].push_back(v);

    }

    int maxi = 0;

    memset(vis, 0);

    memset(path, -1);



    f(i, 1, n+1){



        maxi = max(maxi, dp(i));

    }



    cout<<maxi<<'\n';

    return 0;

}


// Faster C++ solution for the same problem (measured speedup: 3.123691007307995x):
#include<bits/stdc++.h>

#include <ext/pb_ds/tree_policy.hpp>

#include <ext/pb_ds/assoc_container.hpp>



using namespace __gnu_pbds;

using namespace std;





template <typename T>using ordered_set = tree<T, null_type, less<T>,rb_tree_tag, tree_order_statistics_node_update>;



#define PI              acos(-1.0)

#define en '\n'

#define Infi 0x3f3f3f3f

#define LInfi 0x3f3f3f3f3f3f3f3f



#define setzero(a)    memset(a,0,sizeof(a))

#define setneg(a)     memset(a,-1,sizeof(a))

#define setinf(a)     memset(a,126,sizeof(a))





#define READ_IN freopen("Input.txt","r",stdin);

#define READ_OUT freopen("Output.txt","w",stdout);





const int dx[] = {+1,-1,+0,+0};                         // 4 X directions

const int dy[] = {+0,+0,+1,-1};                         // 4 Y directions

const int dx8[] = {+0,+0,+1,-1,-1,+1,-1,+1};            // King X Moves

const int dy8[] = {-1,+1,+0,+0,+1,+1,-1,-1};            // King Y Moves

const int kx[] = {-2, -2, -1, -1,  1,  1,  2,  2};      // Knight X Moves

const int ky[] = {-1,  1, -2,  2, -2,  2, -1,  1};      // Knight Y Moves







int  Set    (int N,int pos) {return N=N | (1<<pos);}    /// turn a bit on

int  reset  (int N,int pos) {return N= N & ~(1<<pos);}  /// turn a bit off

bool check  (int N,int pos) {return (bool)(N & (1<<pos));} /// check, if a bit is on or not



void __print(int x) {cerr << x;}

void __print(long x) {cerr << x;}

void __print(long long x) {cerr << x;}

void __print(unsigned x) {cerr << x;}

void __print(unsigned long x) {cerr << x;}

void __print(unsigned long long x) {cerr << x;}

void __print(float x) {cerr << x;}

void __print(double x) {cerr << x;}

void __print(long double x) {cerr << x;}

void __print(char x) {cerr << '\'' << x << '\'';}

void __print(const char *x) {cerr << '\"' << x << '\"';}

void __print(const string &x) {cerr << '\"' << x << '\"';}

void __print(bool x) {cerr << (x ? "true" : "false");}



template<typename T, typename V>

void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}

template<typename T>

void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}

void _print() {cerr << "]\n";}

template <typename T, typename... V>

void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}

#ifndef ONLINE_JUDGE

#define trace(x...) cerr << "[" << #x << "] = ["; _print(x)

#else

#define trace(x...)

#endif



typedef long long int  ll;

typedef unsigned long long int llu;



vector<vector<int>> graph;



int dp[100005];



int make(int id){



    if(dp[id]!=-1)return dp[id];



    int ans = 0;

    for(int i=0;i<graph[id].size();i++){

        ans = max(ans,1+make(graph[id][i]));

    }

    return dp[id] = ans;

}



int main(void){



    int n,m;

    cin >> n >> m;



    graph.assign(n+2,vector<int>());



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

        int a,b;

        cin >> a >> b;

        graph[a].push_back(b);

    }

    int ans = 0;



    memset(dp,-1,sizeof(dp));



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

        ans = max(ans,make(i));

    }



    cout << ans << endl;



    return 0;

}