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;
using System.Diagnostics;
using System.IO;
using System.Linq;

namespace CompetitiveProgramming
{


  class Solve
  {
    const int INF = (int)(1e9) + 10;
    
    private void Main()
    {
      int N = Scanner.NextInt();
      var a = Scanner.GetLine_Int();

      int ans = 0;
      for (int i = 0; i < N; i++)
      {
        if (a[a[i] - 1] == i + 1) 
        {
          ans++;
        }
      }
      Console.WriteLine(ans / 2);

    }
    public Solve()
    {
      Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
      Main();
      Console.Out.Flush();
    }
  }

  class Program
  {
    static void Main(string[] args)
    {
      new Solve();
    }
  }

  class Scanner
  {
    public static string GetLine() { return Console.ReadLine().Trim(); }
    public static int[] GetLine_Int() { return Console.ReadLine().Trim().Split().Select(int.Parse).ToArray(); }
    public static long[] GetLine_Long() { return Console.ReadLine().Trim().Split().Select(long.Parse).ToArray(); }
    public static ulong[] GetLine_ULong() { return Console.ReadLine().Trim().Split().Select(ulong.Parse).ToArray(); }
    public static string GetContents() { return Console.In.ReadToEnd(); }
    public static int NextInt() { return int.Parse(Console.ReadLine().Trim()); }
    public static long NextLong() { return long.Parse(Console.ReadLine().Trim()); }
    public static ulong NextULong() { return ulong.Parse(Console.ReadLine().Trim()); }
  }
}