#include <bits/stdc++.h>
#define ll unsigned long long
#define MAXN 200
using namespace std;
vector <vector <int> > adj;
vector <bool> visited;
vector <int> parent;
ll tree[MAXN];
ll bin[MAXN];
int fbs(int arr[], int l, int r, int x){
    int res = 0;
    while (l <= r){
        int m = l + (r-l)/2;
        if (arr[m] == x){
            res = m;
            r = m-1;
        }
        else if (arr[m] < x)
            l = m + 1;
        else
             r = m - 1;
        res = l;
    }
    return res;
}
int lbs(int arr[], int l, int r, int x){
    int res = 0;
    while (l <= r){
        int m = l + (r-l)/2;
        if (arr[m] == x){
            res = m;
            l = m+1;
        }
        else if (arr[m] < x)
            l = m + 1;
        else
             r = m - 1;
        res = l;
    }
    return res;
}
void dfs(int s)
{
    visited[s] = true;
    for(int i = 0; i<adj[s].size(); i++)
    {
        if(!visited[adj[s][i]])
        {
            parent[adj[s][i]] = s;
            dfs(adj[s][i]);
        }
    }
}
int bfs(int s)
{
    if(visited[s])
        return 0;
    queue <int> q;
    int ans = 0;
    q.push(s);
    visited[s] = true;
    while(!q.empty())
    {
        s = q.front();
        q.pop();
        for(int i = 0; i<adj[s].size(); i++)
        {
            if(!visited[adj[s][i]])
            {
                visited[adj[s][i]] = true;
                q.push(adj[s][i]);
            }
        }
    }
    return ans;
}
void sieve(int n, vector <int> &primes)
{
    bool prime[n+1];
    memset(prime, true, sizeof(prime));

    for (int p=2; p*p<=n; p++)
    {
        if (prime[p] == true)
        {
            for (int i=p*p; i<=n; i += p)
                prime[i] = false;
        }
    }
    for (int p=2; p<=n; p++)
       if (prime[p])
       {
           primes.push_back(p);
       }
}
int phi(int n)
{
    int result = n;
    for (int p = 2; p * p <= n; ++p) {
        if (n % p == 0) {
            while (n % p == 0)
                n /= p;
            result -= result / p;
        }
    }
    if (n > 1)
        result -= result / n;
    return result;
}
void fin(int n , vector <int> &divs)
{
    for (int i=1; i<=sqrt(n); i++)
    {
        if (n%i == 0)
        {
            if (n/i == i)
                divs.push_back(i);
            else
            {
                divs.push_back(i);
                divs.push_back(n/i);
            }
        }
    }
}
int ct(int x)
{
    int ans = 0;
    while(x>0)
    {
        ans += (x&1);
        x >>= 1;
    }
    return ans;
}
void build(int sti, int lo, int hi)
{
    if(lo == hi)
    {
        tree[sti] = 0;
        bin[sti] = 0;
        return ;
    }
    int left = 2*sti, right = 2*sti+1, mid = (lo+hi)/2;
    build(left, lo, mid);
    build(right, mid+1, hi);
    bin[sti] = bin[left] + bin[right];
    return;
}
void update(int sti, int sl, int sr, int in, int type)
{
    if(sl == sr && sl == in)
    {
        if(type == 1)
        {
            tree[sti] = 2*tree[sti] + 1;
            bin[sti] = ct(tree[sti]);

        }
        else
        {
            tree[sti] /= 2;
            bin[sti] = ct(tree[sti]);
        }
        return ;
    }
    int left = 2*sti, right = 2*sti+1, mid = (sl+sr)/2;
    if(in <= mid)
    {
        update(left, sl, mid, in, type);
    }
    else
    {
        update(right, mid+1, sr, in, type);
    }
    bin[sti] = bin[left] + bin[right];
}
int query(int sti, int sl, int sr, int lo, int hi)
{
    if(sl == lo && sr == hi)
    {
        return bin[sti];
    }
    int mid = (sl+sr)/2;
    int left = 2*sti, right = 2*sti + 1;
    if(hi<=mid)
    {
        return query(left, sl, mid, lo, hi);
    }
    else if(lo>mid)
    {
        return query(right, mid+1, sr, lo, hi);
    }
    else
    {
        return query(left, sl, mid, lo, mid)
             + query(right, mid+1, sr, mid+1, hi);
    }
}
int dp[100005];
int solve(int a, vector <vector<int>> &pa)
{
    if(dp[a] != -1)
    {
        return dp[a];
    }
    int ans = 0;
    for(int i = 0; i<pa[a].size(); i++)
    {
        ans = max(ans, solve(pa[a][i], pa)+1);
    }
    dp[a] = ans;
    return ans;
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n, m, x, y;
    cin>>n>>m;
    for(int i = 0; i<=n; i++)
    {
        dp[i] = -1;
    }
    vector <vector<int> > pa(n+1, vector <int> ());
    for(int i = 0; i<m; i++)
    {
        cin>>x>>y;
        pa[y].push_back(x);
    }
    int ans = 0;
    for(int i = 1; i<=n; i++)
    {
        ans = max(ans, solve(i, pa));
    }
    cout<<ans<<endl;
    return 0;
}


Measured memory_bytes: 10624