#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair <ll,ll> pi;
typedef vector <ll> vi;

const ld PI =  3.1415926535897932384626433832795L;
const ld eps=1e-9;
const int mod = 1e9 + 7;
const int inf = INT_MAX;
const int N = 10500, M = 100500;


#define F first
#define S second
#define PB push_back
#define rs resize

ll n,m;
ll ans=0;

vi son[N];
vi dp(N,0);
vector <bool> used(N, 0);

ll fun(ll v){
    if (used[v]) return dp[v];

    for(auto e:son[v]){
        dp[v] = max(dp[v],fun(e) + 1);
    }

    return dp[v];
}

int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cout<<fixed<<setprecision(20);
//    freopen("longpath.in", "r", stdin);
//    freopen("longpath.out", "w", stdout);
    cin>>n>>m;
    for(int i=0;i<m;i++)
    {
        ll v,to;
        cin>>v>>to;
        son[v].PB(to);
    }
    for(int i=1;i<=n;i++)
    {
        ans = max(ans, fun(i));
    }
    cout<<ans<<endl;
    return 0;
}