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 Ruby (2.3.3) (judge verdict: Runtime Error)

#include <bits/stdc++.h>
#define int long long
#define range(i,x,y) i=(x);i<(y);i++
#define rep(i,n) for(int i=0,i##_len=n;i<i##_len;i++)
#define all(x) begin(x),end(x)
#define lim(x,r,l) (r<=x&&x<l)
typedef long long ll;
typedef long double ld;
using namespace std;

signed main(){
  int n; cin >> n;
  vector<int> a(n);
  rep(i, n){
    cin >> a[i];
  }
  int res = 0;
  rep(i, n){
    if(a[a[i]-1]-1 == i){
      res ++;
    }
  }
  cout << res/2 << endl;
}