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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>

int N, a[100000];
int flag[100000] = { 0 };
int ans = 0;

int main() {
	scanf("%d", &N);
	for (int i = 0; i < N; ++i) { scanf("%d", &a[i]); }

	// TLE解法
	/*for (int i = 0; i < N - 1; ++i) {
		for (int j = i; j < N; ++j) {
			if (i+1 == a[j] && a[i] == j+1) { ans++; }
		}
	}*/

	for (int i = 0; i < N; ++i) {
		if (i + 1 == a[a[i] - 1]) {
			if (flag[i] == 0 && flag[a[i] - 1] == 0) {
				ans++;
				flag[i] = 1;
				flag[a[i] - 1] = 1;
			}
		}
	}

	printf("%d", ans);

	return 0;
}