Problem:
Score : 200 points

Problem Statement

There are N rabbits, numbered 1 through N.

The i-th (1≤i≤N) rabbit likes rabbit a_i.
Note that no rabbit can like itself, that is, a_i≠i.

For a pair of rabbits i and j (i＜j), we call the pair (i，j) a friendly pair if the following condition is met.

Rabbit i likes rabbit j and rabbit j likes rabbit i.

Calculate the number of the friendly pairs.

Constraints

2≤N≤10^5

1≤a_i≤N

a_i≠i

Input

The input is given from Standard Input in the following format:

N
a_1 a_2 ... a_N

Output

Print the number of the friendly pairs.

Sample Input 1

4
2 1 4 3

Sample Output 1

2

There are two friendly pairs: (1，2) and (3，4).

Sample Input 2

3
2 3 1

Sample Output 2

0

There are no friendly pairs.

Sample Input 3

5
5 5 5 5 1

Sample Output 3

1

There is one friendly pair: (1，5).

Solution in PHP (5.6.30) (judge verdict: Accepted)

<?php
/**
 * Created by PhpStorm.
 * User: ramia
 * Date: 2016/09/24
 * Time: 21:00
 */
class STD {
	static function input(){
		$line = trim(fgets(STDIN));
		return $line;
	}

	static function output( $text ){
		echo $text."\n";
	}
}


$n = STD::input();
$neko = explode(' ',STD::input());

$likes[] = 99999;
foreach ( $neko as $hoge ){
	$likes[] = $hoge;
}

$result = 0;
foreach ( $likes as $key => $value ){
	if( $key == 0 ){
		continue;
	}
	//echo $key.' + '.$likes[$value]."\n";
	if( $key == $likes[$value] ){
		$result++;
	}
}

STD::output($result/2);


?>