import java.util.*;

public class Main {
    public static int getDepth(Node node) {
        if (node == null) {
            return 0;
        }
        if (node.childs.size() == 0) {
            return 1;
        }
        int maxDepth = 0;
        for (Node child : node.childs) {
            int depth = 1 + getDepth(child);
            if (depth > maxDepth) maxDepth = depth;
        }
        return maxDepth;
    }

    public static int predators(ArrayList<Integer> datas) {
        ArrayList<Node> nodes = new ArrayList<>();
        ArrayList<Node> parents = new ArrayList<>();

        for (int i = 0; i < datas.size(); i++) {
            nodes.add(new Node(i));
        }

        for (int i = 0; i < datas.size(); i++) {
            int data = datas.get(i);
            Node node = nodes.get(i);

            if (data == -1) {
                parents.add(node);
            }
            else {
                Node parent = nodes.get(data);
                parent.childs.add(node);
                node.parent = parent;
            }
        }

        int maxDepth = 0;
        for (Node parent : parents) {
            int depth = getDepth(parent);
            if (depth > maxDepth) maxDepth = depth;
        }
        return maxDepth;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        ArrayList<Integer> data = new ArrayList<>();
        while (sc.hasNext()) {
            int n = sc.nextInt();
            data.add(n);
        }

        System.out.println(predators(data));
    }
}

class Node {
    Node parent;
    int data;
    ArrayList<Node> childs;

    public Node(int data) {
        this.data = data;
        childs = new ArrayList<>();
    }
}