#pragma GCC optimze("Ofast")
#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define mod 1000000007
#define md 998244353
#define trace(...)
#define ll long long int
#define ld long double
#define fi first
#define se second
#define fr(i,a,b) for(ll i=a;i<=b;i++)
#define frr(i,b,a) for(ll i=b;i>=a;i--)
#define mem(a) memset(a, 0, sizeof(a))
#define vpi std::vector<pair<int,int>>
#define pii pair<int,int>
#define pll pair<ll,ll> 
#define pb push_back
#define sall(v) sort(v.begin(),v.end())
#define all(v) (v.begin(),v.end())
#define minf -10000000
#define vi vector<int>
#define CHMIN(a,b) a=min((a),(b))
#define CHMAX(a,b) a=max((a),(b))
const ll infl=0x3f3f3f3f3f3f3f3fLL;
const int infi=0x3f3f3f3f;
typedef tree<pii, null_type, less<pii>, rb_tree_tag, tree_order_statistics_node_update> oset;
auto clk=clock();
mt19937_64 rang(chrono::high_resolution_clock::now().time_since_epoch().count());
int rng(int lim) {
    uniform_int_distribution<int> uid(0,lim-1);
    return uid(rang);
}
ll power(ll x, ll y,ll p) { 
    ll res = 1;      // Initialize result 
    x = x % p;  // Update x if it is more than or equal to p 
    while (y > 0) {  
        if (y & 1) // If y is odd, multiply x with result
            res = (res*x) % p;  
        y = y>>1; // y = y/2 // y must be even now
        x = (x*x) % p;   
    } 
    return res; 
}
ll power(ll x, ll y) { 
    ll res = 1;      // Initialize result 
    while (y > 0) {    
        if (y & 1)// If y is odd, multiply x with result 
            res = (res*x) ;  
        y = y>>1; // y = y/2 // y must be even now 
        x = (x*x);   
    } 
    return res; 
}
 
ll max3(ll a,ll b,ll c){
    return max(a,max(b,c));
}
ll min3(ll a,ll b,ll c){
    return min(a,min(b,c));
}
bool isPrime(ll n) { // Corner cases 
    if (n <= 1) 
        return false; 
    if (n <= 3) 
        return true; 
    if (n % 2 == 0 || n % 3 == 0) 
        return false; 
    for (ll i = 5; i * i <= n; i = i + 6) 
        if (n % i == 0 || n % (i + 2) == 0) 
            return false; 
    return true; 
}
const int d4i[4]={-1, 0, 1, 0}, d4j[4]={0, 1, 0, -1};
const int d8i[8]={-1, -1, 0, 1, 1, 1, 0, -1}, d8j[8]={0, 1, 1, 1, 0, -1, -1, -1};
const int nax=1e5+10;
int n,m;
vector<int> A[nax];
int dp[nax];
bool visited[nax];
void dfs(int idx){
    visited[idx]=1;
    for(auto x:A[idx]){
        if(!visited[x])dfs(x);
        dp[idx]=max(1+dp[x],dp[idx]);
    }
}
void solve(){
    cin>>n>>m;
    fr(i,1,n){dp[i]=0;A[i].clear();visited[i]=0;}
    int a,b;
    fr(i,1,m){
        cin>>a>>b;
        A[a].pb(b);
    }
    fr(i,1,n){
        if(!visited[i])dfs(i);
    }
    int ans=0;
    fr(i,1,n)ans=max(ans,dp[i]);
    cout<<ans<<endl;
    return;
}

int main(){
    ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    srand(chrono::high_resolution_clock::now().time_since_epoch().count());
    cout<<fixed<<setprecision(8);
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    #endif
    int t;
    // cin>>t;
    t=1;
    while(t--){
        solve();
    }
    #ifndef ONLINE_JUDGE
    cout<<endl<<endl<<endl<<endl<<"Time elapsed: "<<(double)(clock()-clk)/CLOCKS_PER_SEC<<endl;
    #endif
    return 0;
}

Measured memory_bytes: 12216