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

#include <algorithm>

#include <iostream>

#include <iomanip>

#include <cstdio>

#include <cstdlib>

#include <cmath>

#include <cstring>

#include <vector>

#include <string>

#include <climits>

#include <map>

#include <set>

#include <stack>

#include <queue>

#include <deque>

#include <stdio.h>

//#define int long long

using namespace std;

#define loop(n) for(int i=0;i<n;i++)

#define endl '\n'

#define OnlineJudgec

#define F first

#define S second

#define  precision(n)  cout << fixed << setprecision(n)

#define clr(v,d ) memset(v, d, sizeof(v))

typedef long long ll;

typedef vector<int> vi;

void init()

{

    cin.tie(0);

    cin.sync_with_stdio(0);

    cout.tie(0);

    #ifdef OnlineJudge

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

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

    #endif

}

ll S(ll x){ll s = 0;while(x){s += x % 10;x /= 10;}return s;} /// sum of digits.

ll powe(ll a , ll n){if(n == 1)return a;return a * powe(a ,n-1);}

ll com(int n,int m){ll z = 1;for(int i = 0; i < m;i++){z = z * ( n - i) / (i + 1) ;}return z;}

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

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

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

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

/// <<------------------------------------------------------------------------------>>

const int N = 100009;

int n,m;

int MOD = int(1e9) + 7;

int dp[N];

vector<vi>g(N);

int dfs(int node)

{

    int &rst = dp[node];

    if(~rst)

        return rst;

    rst  = 0;

    for(int neighbor : g[node])

    {

        rst =max(rst , 1 + dfs(neighbor));

    }

    return rst;

}

/// <<------------------------------------------------------------------------------>>

int main()

{

    init();

    clr(dp,-1);

    cin >> n >> m;

    loop(m)

    {

        int l ,r;

        cin >> l >> r;

        g[l].push_back(r);

    }

    int ans = 0;

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

    {

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

    }

    cout<<ans<<endl;

}


// Faster C++ solution for the same problem (measured speedup: 1.1209625561703682x):
#include <bits/stdc++.h>

#include <set>

using namespace std;

#define mp             make_pair

#define pb             push_back

#define fi             first

#define se             second

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

#define fill(x, y)     memset(x, y, sizeof(y))

#define all(x)         (x).begin(), (x).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<iii>    viii;



int n;

int opt;

bool visited[100010];

vector<int> graph[100010];

int in[100010];

int dp[100010];



void dfs(int u) {

    visited[u] = true;

    for (int v : graph[u]) {

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

        in[v]--;

        if (in[v] == 0) dfs(v);

    }

}



int main() {

  ios::sync_with_stdio(false);

  cin.tie(0);

  int m;

  cin >> n >> m;

  int a, b;

  opt = 0;

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

      cin >> a >> b;

      graph[a].pb(b);

      in[b]++;

  }

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

      if (!visited[i] && in[i] == 0) dfs(i);

  }

  int opt = 0;

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

      opt = max(opt, dp[i]);

  }

  cout << opt << "\n";

  return 0;

}