#include <bits/stdc++.h>
using namespace std;
#define fio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define mod 1000000000007
#define MAXN 100005
#define ll long long
#define pb push_back
#define sortv(v) sort(v.begin(),v.end())
#define tc(t) int t;cin>>t;while(t--)
#define copy copy_n
#define fr(i,a,b) for(long long i=a;i<b;i++)
#define frr(i,a,b) for(long long i=a;i>=b;i--)
#define frn(i,a,b,n) for(long long i=a;i<=b;i=i+n)
#define gcd(x,y) __gcd(x,y)
#define ld long double
#define um unordered_map
#define mkp make_pair
#define b_s binary_search
#define des greater<ll>()
#define fi first
#define se second
#define PI 3.1415926535
#define FP(n) fixed<<setprecision(n)
ll mul(ll a, ll b, ll p = mod) {return ((a % p) * (b % p)) % p;}
ll add(ll a, ll b, ll p = mod) {return (a % p + b % p) % p;}
void input(ll a[], ll sz) {fr(i, 0, sz) cin >> a[i];}
void print(ll a[], ll sz) {fr(i, 0, sz) {if (i == sz - 1) cout << a[i] << "\n"; else cout << a[i] << " ";}}
ll maxr(ll a[], ll sz) {ll ma; fr(i, 0, sz) {if (i == 0) ma = a[i]; else if (a[i] > ma) ma = a[i];} return ma;}
ll minr(ll a[], ll sz) {ll mi; fr(i, 0, sz) {if (i == 0) mi = a[i]; else if (a[i] < mi) mi = a[i];} return mi;}
ll isprm(ll n) {
    if (n <= 1)  return 0;
    if (n <= 3)  return 1;
    if (n % 2 == 0 || n % 3 == 0) return 0;

    for (int i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0)
            return 0;
    return 1;
}
/*ll spf[MAXN];
void sieve()
{
    spf[1]=1;
    for (ll i=2; i<MAXN; i++)
        spf[i] = i;
    for (ll i=4; i<MAXN; i+=2)
        spf[i] = 2;

    for(ll i=3;i*i<MAXN;i++)
    {
        if (spf[i]==i)
        {
            for (int j=i*i;j<MAXN;j+=i)
                if (spf[j]==j)
                    spf[j]=i;
        }
    }
}*/
ll power(ll x, ll y, ll p = mod)
{
    ll res = 1;
    x = x % p;
    while (y > 0)
    {
        if (y & 1)
            res = (res * x) % p;
        y = y >> 1; // y = y/2
        x = (x * x) % p;
    }
    return res;
}
ll modInverse(ll n, ll p = mod)
{
    return power(n, p - 2, p);
}
ll ncrMod(ll n, ll r, ll p = mod)
{
    if (r == 0)
        return 1;
    ll fac[n + 1];
    fac[0] = 1;
    for (ll i = 1 ; i <= n; i++)
        fac[i] = fac[i - 1] * i % p;
    return (fac[n] * modInverse(fac[r], p) % p *
            modInverse(fac[n - r], p) % p) % p;
}
//a+b=xor(a,b)+2*(a&b)//
struct comp
{
    bool operator()(const pair<ll, ll> &a,
                    const pair<ll, ll> &b)const
    {
        if (a.fi == b.fi)
        {
            return (a.se > b.se); //second elem in descending
        }
        return (a.first < b.first);//first elem in ascending
    }
};

ll n,m,vis[100005],dp[100005];
vector<ll>a[100005];

ll dfs(ll u)
{
    vis[u]=1;
    ll k=0;
    if(dp[u]!=-1)
    return dp[u];
    for (int i=0; i<a[u].size(); i++)
    {
         k=max(k,1+dfs(a[u][i]));
    }
    dp[u]=k;
    return dp[u];
}








int main()
{
    fio;
    cin>>n>>m;
    fr(i,0,m)
    {
        ll x,y;
        cin>>x>>y;
        a[x].pb(y);
    }
    fr(i,0,100005)
    {
    vis[i]=0;
    dp[i]=-1;
    }
    fr(i,1,n+1)
    {
        if(vis[i]==0)
        dfs(i);
    }
    ll ans=0;
    fr(i,1,n+1)
    {
    //cout<<dp[i]<<" ";
    ans=max(ans,dp[i]);
    }
    //cout<<"\n";
    cout<<ans<<"\n";
    return 0;
}


Measured memory_bytes: 9344