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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

#define maxsiz 1000000
#define F first
#define S second
#define fr(i,k,n) for(int i = k ; i < n ; i++ )
#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define printvect(a,n) fr(i,0,n) cout << a[i] << " " ;
#define point pair<int,int>
#define pii pair<int,int>
#define pib pair<int,bool>
#define arr vector<int> 
#define vectin(a,n) fr(i,0,n) { cin >> a[i] ; }
typedef unsigned long long int ull;

int main()
{
	int n;
	cin >> n ; 
	vector<int> a(n);
	fr(i,0,n)
		cin >> a[i] ; 

	int count = 0 ;
	fr(i,0,n)
	{
		if(a[a[i]-1]==i+1)
			count++;
	}
	cout << count/2 << endl;
}