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.io.InputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.BufferedReader;

class MyScanner{
    InputStream stream;
    public MyScanner(){
        stream = System.in;
    }
    
    public int nextInt(){
        int ret = 0;
        try{
            while(true){
                char readed = (char)stream.read();
                if(readed < '0' || readed > '9'){
                    break;
                }
                ret = ret * 10 + (readed - '0');
            }
        }catch(IOException e){
            e.printStackTrace();
        }finally{
            return ret;
        }
    }
}

class Main {
    public static void main(String args[]) {
        MyScanner sc = new MyScanner();
        int n = sc.nextInt();
        int[] a = new int[n];
        for(int i = 0; i < n; ++i){
            a[i] = sc.nextInt() - 1;
        }
        
        int ret = 0;
        for(int i = 0; i < n; ++i){
            if(a[a[i]] == i){
                ++ret;
            }
        }
        System.out.println(ret / 2);
    }
}