A company has n employees numbered from 1 to n . Each employee either has no immediate manager or exactly one immediate manager, who is another employee with a different number. An employee A is said to be the superior of another employee B if at least one of the following is true: Employee A is the immediate manager of employee B Employee B has an immediate manager employee C such that employee A is the superior of employee C . The company will not have a managerial cycle. That is, there will not exist an employee who is the superior of hisher own immediate manager. Today the company is going to arrange a party. This involves dividing all n employees into several groups: every employee must belong to exactly one group. Furthermore, within any single group, there must not be two employees A and B such that A is the superior of B . What is the minimum number of groups that must be formed?

输入描述:

The first line contains integer n (1 ≤ n ≤ 2000) — the number of employees.The next n lines contain the integers pi (1 ≤ pi ≤ n or pi = -1). Every pi denotes the immediate manager for the i-th employee. If pi is -1, that means that the i-th employee does not have an immediate manager. It is guaranteed, that no employee will be the immediate manager of himherself (pi ≠ i). Also, there will be no managerial cycles.

输出描述:

Print a single integer denoting the minimum number of groups that will be formed in the party.

备注:

For the first example, three groups are sufficient, for example: Employee 1 Employees 2 and 4 Employees 3 and 5