DFS

#include <bits/stdc++.h>

using namespace std;
const int N = 2025;

int n, p[N], vis[N], ans;
vector<int> g[N];
void dfs(int i, int dep) {
    ans = max(ans,dep);
    vis[i] = 1;
    for(int j:g[i]) if(!vis[j]) dfs(j, dep+1);
}
int main() {
    cin >> n;
    for(int i = 1; i <= n; i++) cin >> p[i];
    for(int i = 1; i <= n; i++) {
        if(p[i] != -1) g[p[i]].push_back(i);
    }
    for(int i = 1; i <= n; i++) if(!vis[i]) dfs(i,1);
    cout << ans << '\n';
}