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: Wrong Answer)

import java.util.*;

public class Main {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int[] rb = new int[N + 1];
        int[] ht = new int[N];
        for (int i = 0; i < N; ++i) {
            rb[i + 1] = sc.nextInt();
        }
        sc.close();

        int res = 0;
        for (int i = 1; i <= N; ++i) {
            if (ht[i - 1] == 1 && ht[rb[i] - 1] ==  1) continue;
            if (i == rb[rb[i]]) {
                res++;
            }
            ht[i - 1] = 1;
            ht[rb[i] - 1] = 1;
        }

        System.out.println(res);
    }
}