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: Wrong Answer)

#include <string>
#include <vector>
#include <map>
#include <set>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <sstream>
#include <list>

using namespace std;


int main(int argc, char * argv [] ) {
    int N = 0;
    cin >> N;

    vector<int> rabits(N + 1);
    for (int i = 1; i <= N; ++i) {
        cin >> rabits.at(i);
    }
    int cnt = 0;
    for (int i = 1; i <= N; ++i) {
        int love1 = rabits.at(i);
        if (love1 != -1) {
            int love2 = rabits.at(love1);
            if (love2 == i) {
                ++cnt;
            }
            rabits.at(i) = -1;
            rabits.at(love1) = -1;
        }
    }
    cout << cnt << endl;
	return 0;
}