// Slower C++ solution:
//#pragma GCC target ("avx2,sse2")

//#pragma GCC optimization ("Ofast")

//#pragma GCC optimization ("unroll-loops")

#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;



#define ordered_set tree <pair <ll, int>, null_type, less <pair <ll, int> >, rb_tree_tag, tree_order_statistics_node_update>

#define ll long long

#define ull unsigned long long

#define db long double

#define pb push_back

#define pf push_front

#define ppb pop_back

#define ppf pop_front

#define fi first

#define se second

#define mp make_pair

#define up_b upper_bound

#define low_b lower_bound

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

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

#define endl "\n"



#define left fsdsdfoisf

#define sum dpsdfioppsf

#define assign xcvjlkdjfio

#define trie fksdfkjkfnjuiv

#define next sidlfjsfkl

#define merge sdfksdkfsldf



//#define int long long



using namespace std;



void dout() {

    cerr << endl;

}

template <typename Head, typename... Tail>

void dout(Head H, Tail... T) {

    cerr << H << ' ';

    dout(T...);

}



//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

typedef pair <int, int> pii;



const int N = 1e5 + 7;

int n, m, dp[N];

vector <int> q[N], r[N], ord;

bool used[N];



void dfs(int v) {

    used[v] = true;

    for (auto i : q[v]) {

        if (!used[i]) {

            dfs(i);

        }

    }

    ord.pb(v);

}



void solve(int tc) {

    // check for (int i = 0; i < n; j++)

    cin >> n >> m;

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

        int x, y;

        cin >> x >> y;

        q[x].pb(y);

        r[y].pb(x);

    }

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

        if (!used[i]) {

            dfs(i);

        }

    }

    reverse(all(ord));

    int res = 0;

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

        int v = ord[i - 1];

        for (auto j : r[v]) {

            dp[v] = max(dp[v], dp[j] + 1);

        }

        res = max(res, dp[v]);

    }

    cout << res;

}



signed main() {

    ios_base::sync_with_stdio(false);

    cin.tie(NULL);

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

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

    int tc = 1;

//    cin >> tc;

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

        solve(i);

//        cleanup();

    }

}


// Faster C++ solution for the same problem (measured speedup: 6.332413549144222x):
#include <bits/stdc++.h>



using namespace std;



// Header files, namespaces,

// macros as defined above

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;



#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>





#define f first

//#define s second

#define db long double

#define pb push_back

#define sz(c) (c).size()

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



typedef long long ll;

typedef pair <int , int> ii;

typedef pair <ii , int> iii;

typedef vector <int> vi;

typedef vector <ii> vii;

typedef vector <vi> vvi;

typedef vector <vii> vvii;



const ll mod = 998244353;

const int MAX = 1e5+5;

const db eps = 1e-13;



bool mk[MAX];

int dp[MAX];



vvi g;



int dfs_dp( int node ) {



    mk[node] = true;

    int mx = 0;

    for( int nw : g[node] ) {



        if( !mk[nw] )

            mx = max( dfs_dp(nw) , mx );

        else

            mx = max( dp[nw] , mx );

    }



    return dp[node] = mx+1;

}







int main()

{

	ios_base::sync_with_stdio(0);

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

    cout<<setprecision(0)<<fixed;

    int ans = 0;

    int n , m;cin>>n>>m;

    vi v;

    g = vvi(n+1,v);

    while(m--) {



        int a, b;

        cin>>a>>b;

        g[a].pb(b);



    }



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

        if( !mk[i] )

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



    cout<<ans-1<<"\n";



}