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;
using System.Collections.Generic;
using System.Collections;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace tes
{
	class contest
	{
		static void Main(string[] args)
		{
			var num = int.Parse(Console.ReadLine());
            var input = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();

            var memo = new bool[num+1];
            var set = new HashSet<Tuple<int, int>>();

            for(int i = 0; i< input.Length; i++)
            {
                int tmp = input[i];
                int now = i + 1;

                set.Add(new Tuple<int, int>(tmp, now));
            }

            int count = 0;
            foreach(var item in set)
            {
                if (set.Contains(new Tuple<int, int>(item.Item2, item.Item1)) && !memo[item.Item2])
                {
                    memo[item.Item1] = true;
                    count++;
                }
            }

            Console.WriteLine(count);
		}		
		 
	}
}