// 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: 1.6163127917982587x):
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

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

#define INF 1e18

int n, m;

vector<int> graph[100020];

vector<int> dp(100020, -1);

vector<bool> visited(100020, false);

int dfs(int node, int sum)

{

    if(dp[node] != -1)

        return dp[node];

    if(graph[node].empty())

        return 1;

    else{

        int temp, max = 0;

        for (auto child : graph[node]) {

            temp = dfs(child, sum+1);

            if(temp > max)

                max = temp;

        }

        return dp[node] = max+1;

    }

}

int check()

{

    int mx = 0;

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

        if(!visited[i])

            mx = max(mx, dfs(i, 0));

    }

    return mx-1;

}

int main() {

    fast;

    cin>>n>>m;

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

        int a, b;

        cin>>a>>b;

        graph[a].push_back(b);

    }

    cout<<check();

}