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.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class Main {

    public static void main(String[] args) throws IOException {
        
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        PrintWriter pr = new PrintWriter(System.out);
        int N = Integer.parseInt(br.readLine());
        
        String s = br.readLine();
        StringTokenizer st = new StringTokenizer(s);
        int [] RABBIT = new int[N];
        for(int i=0;i<RABBIT.length;i++){
            RABBIT[i]=Integer.parseInt(st.nextToken());
        }
        
        int [] RABBITLIKE = new int [N];
        for(int k=0;k<RABBITLIKE.length;k++){
            if(RABBITLIKE[RABBIT[k]-1]==0)
                RABBITLIKE[RABBIT[k]-1] = k+1;
        }
        
        int count =0;
        for(int i=0;i<RABBIT.length;i++){
            if(RABBIT[i]==RABBITLIKE[i]){
                count++;
            }
        }
        
        pr.println(count/2);
        pr.close();
        
    }
    
}