#include <bits/stdc++.h>

using namespace std;

#define pb push_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
#define all(a) (a).begin(),(a).end()
#define endl "\n"

#define aim_cpo do{ios::sync_with_stdio(false);\
cin.tie(nullptr);cout.precision(10);cout<<fixed;}while(0)

#define LOCAL_INPUT do{FILE *stream1;\
stream1=freopen("/Users/aim_cpo/CLionProjects/competitive/in.txt","r",stdin);\
if(stream1==nullptr)return 0;}while(0)

#define LOCAL_OUTPUT do{FILE *stream2;\
stream2 = freopen("out.txt", "w", stdout);\
if (stream2 == nullptr) return 0;}while(0)

#ifdef LOCAL_DEFINE
#define show(x) cerr << #x << " = " << x << endl
#else
#define show(x)
#endif

typedef long long ll;
typedef unsigned long long ull;

constexpr const int INT_INF=0x3f3f3f3f; // 1061109567
constexpr const ll LL_INF=0x3f3f3f3f3f3f3f3f; // 4557430888798830399

template <typename T> bool chmin(T &a, T b) {
    if (a > b) {a = b;return true;}return false;
}

template <typename T> bool chmax(T &a, T b) {
    if (a < b) {a = b;return true;}return false;
}

// INT
#define GCD(a, b) __gcd(a, b)
template <typename T> T LCM(T a, T b) {return a / GCD(a, b) * b;}
template <typename T> T EXTGCD(T a, T b, T& x, T& y) {
    T d = a;
    if (b != 0) {d=EXTGCD(b,a%b,y,x);y-=(a/b)*x;}
    else x=1,y=0;
    return d;
}
template <typename T> bool is_prime(T a) {
    for(int i=2;i*i<=a;i++)if(a%i==0)return 1;
    return 0;
}

// MOD
const ll MOD = (ll)1e9 + 7;
#define add(a, b) ((a % MOD) + (b % MOD)) % MOD
#define mul(a, b) ((a % MOD) * (b % MOD)) % MOD
#define sub(a, b) ((a % MOD) + MOD - (b % MOD)) % MOD
template <typename T> T mod_inverse(T a, T mod, bool prime){ // if mod is prime, "prime" is true.
    if(prime){
        T tmp=mod-2,now=a,res=1;while(tmp){if(tmp&1)res=mul(res,now);now=mul(now,now);tmp>>=1;}
        return res;
    }else{T x,y;EXTGCD(a,mod,x,y);return (mod+x%mod)%mod;}
}
#define divide(a, b) ((a % MOD) * (mod_inverse(b, MOD, true))) % MOD

//LLの数値をつかう時は最後にLをつける癖をつけよう
//
//  ┏┓┏┓　┓　┏┓
//  ┏┛┃┃　┃　┗┫
//  ┗┛┗┛　┻　┗┛
//
//   謹┃賀┃新┃年┃
//   ━┛━┛━┛━┛
//WWWWWWWWWWWWWWWWWWWWWW

int n, k, m;
string s, t;
vector <int> v[100001];
ll dp[100001];
int h[100001];

int main() {
    aim_cpo; // インタラクティブのときは消すように.
#ifdef LOCAL_DEFINE
    LOCAL_INPUT; // インタラクティブのときは消すように.
//    LOCAL_OUTPUT; // ファイルに出力したいときのみ
    show(MOD);
#endif
    cin >> n >> m;
    for (int i = 0; i < m; i++) {
        int from, to; cin >> from >> to;
        from--; to--;
        v[from].push_back(to);
        h[to]++;
    }
    stack <int> st;
    for (int i = 0; i < n; i++) {
        if (h[i] == 0) st.push(i);
    }
    vector <int> res;
    while (!st.empty()) {
        int i = st.top(); st.pop();
        res.push_back(i);
        for (int j = 0; j < (int)v[i].size(); j++) {
            h[v[i][j]]--;
            if (h[v[i][j]] == 0) st.push(v[i][j]);
        }
    }
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < (int)v[res[i]].size(); j++) {
            dp[v[res[i]][j]] = max(dp[v[res[i]][j]], dp[res[i]] + 1);
        }
    }
    ll ans = 0;
//    for (int i = 0; i < n; i++) {
//        cerr << i << " = " << dp[i] << " ";
//    }
    for (int i = 0; i < n; i++) {
        chmax(ans, dp[i]);
    }
    cout << ans << endl;
#ifdef  LOCAL_DEFINE
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << "s." << endl;
#endif
    return 0;
}

Measured memory_bytes: 7416