#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 2e3;
vector<int> e[MAX_N], d;
vector<char> flag;
void dfs(int v) {
  if (d[v] != -1) return;
  d[v] = 1;
  for (int to : e[v]) {
    dfs(to);
    d[v] = max(d[v], d[to] + 1);
  }
}
int main() {
  int n;
  scanf("%d", &n);
  d.assign(n, -1);
  for (int i = 0; i < n; i++) {
    int x;
    scanf("%d", &x);
    if (x != -1) e[x - 1].push_back(i);
  }
  for (int i = 0; i < n; i++)
    if (d[i] == -1) dfs(i);
  int ans = 1;
  for (int i : d) ans = max(ans, i);
  printf("%d", ans);
  return 0;
}
