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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;

class CF2016QAB
{
	static public void Main(string[] args)
	{
		int N = int.Parse(Console.ReadLine());
		string[] vals = Console.ReadLine().Split(' ');
		int[] a = new int[N+1];
		for(int i=1; i <= N; i++)
		{
			a[i] = int.Parse(vals[i-1]);
		}

		int ans = 0;
		bool[] check = new bool[N+1];
		for(int i=1; i<=N; i++)
		{
			if(check[i] == false && a[a[i]] == i)
			{
				ans++;
				check[a[i]] = true;
			}
		}
		Console.WriteLine(ans);
	}
}