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;
using Graph = vector<vector<int>>;
#define rep(i,a,b) for (int i = (a); i < (b); i++ )
typedef pair<int,int> P;
typedef long long ll;
const int INF = 100000000;
const double PI=3.14159265358979323846;

int main (){
    int N;
    cin >> N;
    vector<vector<int>> a(N);
    int b[N];
    rep(i,0,N) {
        int tmp;
        cin >> tmp;
        b[i] = tmp-1;
        a[tmp-1].push_back(i);
    }

    int ans = 0;

    rep(i,0,N) {
        for (auto j:a[i]) {
            if (b[i]==j) {
                ans++;
            }
        }
    }
    ans/=2;
    cout << ans << endl;


}