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 _GLIBCXX_DEBUG
#define rep(i, n) for (int64_t i = 0; i < (int64_t)(n); i++)
using vi = vector<int64_t>;
using vvi = vector<vi>;
int64_t INF = 1000000007;
// int64_t INF = 9223372036854775807;

int main() {
    // fasten cin
    cin.tie(0);
    ios::sync_with_stdio(false);
    // implement
    int64_t n;
    cin >> n;
    vi a(n);
    rep(i,n) cin >> a.at(i);
    int64_t res = 0;
    rep(i,n){
        if(a.at(a.at(i)-1)-1 == i){
            res++;
        }
    }
    cout << res/2 << endl;
}