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: Wrong Answer)

using System;
class Program
 {
  	static void Main(string[] args)
    {
      int a = int.Parse(Console.ReadLine()); //標準入力
      
      string[] str = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得
      int[] b = new int[str.Length];

      for(int i = 0; i < str.Length; i++ )
      {
        b[i] = int.Parse(str[i]);
      }
      
      int nakayoshi = 0;
      int r = 1;
      
      for(int x = 0; x < b.Length; x++)
      {
        if(x + 1 >= b.Length)
        {
          break;
        }
        
        
        if(b[x] != b[r])
        {
          r++;
          nakayoshi++;
        }        
      }
      Console.WriteLine(nakayoshi);
    }
 }