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 Rust (1.15.1) (judge verdict: Accepted)

use std::io;

fn main() {
    let mut s = String::new();
    io::stdin().read_line(&mut s).ok();

    //let n = s.trim().parse().unwrap();

    let mut s = String::new();
    io::stdin().read_line(&mut s).ok();

    let xs: Vec<usize> = s.trim().split(' ').map(|x| x.parse().unwrap()).collect();

    let mut res = 0;
    for (i, &x) in xs.iter().enumerate() {
        if xs[x - 1] == i + 1 {
            res += 1;
        }
    }

    println!("{}", res / 2);
}