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.Linq;

namespace CodeFest2016
{
	class Program
	{
		static void Main(string[] args)
		{
			var debug = false;

			var n = debug ? 4 : int.Parse(Console.ReadLine());

			var rabbits = (debug ? "2 1 4 3" : Console.ReadLine()).Split(" ".ToCharArray()).Select(num => int.Parse(num)-1).ToArray();

			var pairs = 0;
			for (int i = 0; i < n; i++)
			{
				if (rabbits[i] == -1) continue;
				if (i == rabbits[rabbits[i]])
				{
					pairs++;
					rabbits[rabbits[i]] = -1;
					rabbits[i] = -1;
				}
            }

			Console.WriteLine(pairs);
		}
	}
}