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 (GCC 5.4.1) (judge verdict: Accepted)

# include <stdio.h>
//# include <iostream>
//# include <cstdio>
//# include <conio.h>

//using namespace std;

const int N = 100100;

//int n, a [N], i, j, k;

int main ()
{
   int n, a [N], i, j, k;

   scanf ("%d", &n);
   
   for ( i=1; i <= n; i++ )
      scanf ("%d", &a [i]);

   for ( i=1; i <= n; i++ )
   {
      j = a [i];
      if ( a [j] == i )
         k++;
   }

   printf ("%d", k/2);

//getch();
   return 0;
}