#include <bits/stdc++.h>
using namespace std;
#define MAXN 20200
int breadth[MAXN];
vector<int> rev[MAXN];
vector<int> finals;
int arr[MAXN];
int visited[MAXN];
set<pair<int, int>> st;
void dfs(int v) {
auto p=st.find({breadth[v], v});
if(p==st.end()) return;
st.erase(p);
for(auto u : rev[v]) {
dfs(u);
}
}
int main() {
int n;
cin >> n;
for(int i=0;i<n;i++) {
cin >> arr[i];
if(arr[i]<0) finals.push_back(i);
else rev[arr[i]].push_back(i);
}
for(int i=0;i<n;i++) breadth[i]=-1;
queue<int> q;
for(auto v : finals) {
breadth[v]=1;
q.push(v);
}
while(!q.empty()) {
int v=q.front();
q.pop();
for(auto u : rev[v]) {
if(breadth[u]>=0) continue;
breadth[u]=breadth[v]+1;
q.push(u);
}
}
st.insert({0, -1});
for(int i=0;i<n;i++) st.insert({breadth[i], i});
int v=0;
int cur=0;
int ans=0;
memset(visited, 0, sizeof(visited));
while(v!=-1&&!visited[v]) {
visited[v]=1;
dfs(v);
cur++;
auto p=st.end();
p--;
ans=max(ans, cur+p->first);
v=arr[v];
}
cout << ans << endl;
return 0;
}
