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# (.NET Core 3.1.201) (judge verdict: Time Limit Exceeded)

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

namespace AtCoder.Abc
{
    class QuestionB
    {
        public static void Main(string[] args)
        {
            // 整数の入力
            int n = int.Parse(Console.ReadLine());

            // 整数配列の入力
            var a = Console.ReadLine().Split(' ').Select(i => int.Parse(i) - 1).ToArray();

            var rabits = new bool[n][];

            for (int i = 0; i < n; i++) {
                rabits[i] = new bool[n];
            }

            var count = 0;

            for (int i = 0; i < n; i++) {
                if (rabits[i][a[i]]) {
                    count++;
                }

                rabits[i][a[i]] = true;
                rabits[a[i]][i] = true;
            }

            Console.WriteLine(count);
        }
    }
}