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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

const mod = 1000000007

// 1MB
const ioBufferSize = 1 * 1024 * 1024

var sc = func() *bufio.Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Buffer(make([]byte, ioBufferSize), ioBufferSize)
	sc.Split(bufio.ScanWords)
	return sc
}()

func next() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func main() {
	n := nextInt()
	a := make([]int, n)

	for i := 0; i < n; i++ {
		a[i] = nextInt() - 1
	}

	res := 0
	for i := 0; i < n; i++ {
		if i == a[a[i]] {
			res++
		}
	}

	fmt.Println(res / 2)
}