/* aman36 - Aman Raj */

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace std;

#define fast_io ios :: sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define AfterDecimal(n) cout << fixed << setprecision(n);
#define pb push_back
#define ll long long
#define ld long double
#define all(a) a.begin(), a.end()
#define pll pair<ll, ll>
#define pii pair<int, int>
#define F first
#define S second
#define bits(x) __builtin_popcount(x)
#define fori(n)  for (ll i = 0; i < n; i++)
#define forj(n)  for (ll j = 0; j < n; j++)
#define fork(n)  for (ll k = 0; k < n; k++)
#define forlp(k,x,n)  for (ll k = x; k <= n; k++)
#define vll vector <ll>
#define mll map <ll, ll>
#define vpll vector <pll>
#define vi vector <int>
#define mem(x, v) memset(x, v, sizeof(x));
#define int ll
#define endl "\n"

// priority_queue<ll, vector<ll>, less<ll>>pq;     // Max Heap
// priority_queue<ll, vector<ll>, greater<ll>>pq;  // Min Heap

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

#ifdef DEBUG
#define debug(...) __f(#__VA_ARGS__, __VA_ARGS__)
template<typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cout << "# "<< name << " = " << arg1 << '\n';
}
template<typename Arg1, typename... Args>
void __f(const char* names, Arg1 && arg1, Args &&... args){
        const char* comma = strchr(names + 1 ,',');
        cout << "# ";
        cout.write(names, comma - names)<< " = " << arg1 << " | ";
        __f(comma + 1, args...);
}
#else
#define debug(...) 36
#endif

ll power(ll x, ll y, ll p) { 
    ll res = 1;
    x = x % p;  
    while (y > 0) {
        if (y & 1) 
            res = (res * x) % p; 
        y = y >> 1;
        x = (x * x) % p; 
    }
    return res;
}

bool isPrime(int n) 
{
    if (n <= 1) return false; 
    if (n <= 3) return true; 
    if (n % 2 == 0 || n % 3 == 0) return false; 
    for (int i = 5; i * i <= n; i = i + 6) 
        if (n % i == 0 || n % (i + 2) == 0) return false; 
    return true; 
}

const ll INF = (ll)(8e18);
const ll MOD = 1e9 + 7;
const ll MOD2 = 998244353;
const int M = 4000+5;
const int N = 1e5+5;
const int maxn = 1e5+5;

vll g[N];

void solve() {
    int n, m;
    cin >> n >> m;

    int indeg[n+1] = {0};
    fori(m) {
        int x, y;
        cin >> x >> y;
        g[x].pb(y);
        indeg[y]++;
    }

    queue<int>q;
    for(int i=1; i<=n; i++) {
        if(!indeg[i]) q.push(i);
    }

    int dp[n+1] = {0};

    while(!q.empty()) {
        int tp = q.front();
        q.pop();

        for(auto d: g[tp]) {
            dp[d] = max(dp[d], dp[tp] + 1);
            indeg[d]--;
            if(!indeg[d]) q.push(d);
        }
    }

    cout << *max_element(dp, dp+n+1) << endl; 
}

signed main() {
    fast_io;
    // #ifndef ONLINE_JUDGE
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    // #endif

    int tc = 1;
    // cin >> tc;

    for(int i=1; i<=tc; i++) {
        // cout << "Case #" << i << ": " ;
        solve();
    }

    #ifdef LOCAL
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
    #endif

    return 0;
}

// read the question correctly
// read code carefully
// corner cases // int vs ll // cin vs scanf // clear structures // maths
// check if max, min set to INT_MAX instead of INF
// check constraint 

Measured memory_bytes: 10524