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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.Scanner;

public class Main {
    public static void main(String [] args) {
    	Scanner sc = new Scanner(System.in);
    	int n = sc.nextInt();
    	int[] a = new int[n + 1];
    	int i = 1;
    	while(i <= n) {
    		a[i] = sc.nextInt();
    		i++;
    	}
    	int count = 0;
    	for (i = 1; i <= n; i++) {
    		int ai = a[i]; // 2
    		int aj = a[ai]; // 1
    		if (i == aj) {
    			count ++;
    		}
    		// 2 1 4 3
    		// 1 2 3 4
    		
    	}
    	System.out.println(count/2);
    }
}