Expedition

An expedition is being planned to a neighboring star system. There are $n$ candidates, numbered from 1 to $n$, from whom the expedition members must be chosen. The organizers want to send as many candidates as possible.

The candidates were surveyed, and each could name at most one other candidate with whom they are not willing to go on the expedition. The survey result for candidate $i$ is an integer $a_i$, equal to the number of a candidate with whom candidate $i$ is not willing to go, or $-1$ if candidate $i$ is willing to go in any group.

Now the organizers must decide who will go on the expedition. They decided to choose the members so that if candidate $i$ is included and $a_i \ne -1$, then candidate $a_i$ is not included. The organizers want to choose the largest possible number of expedition members.

Write a program that, given the survey results, determines the maximum number of candidates that can be sent on the expedition.

The first line contains the integer $n$, the number of candidates ($1 \le n \le 300\,000$).

The next $n$ lines contain the survey results; the $i$-th of these lines contains the result of candidate $i$, an integer $a_i$ ($a_i = -1$ or $1 \le a_i \le n$, $a_i \ne i$).

Output a single integer on one line: the maximum number of candidates that can be sent on the expedition.