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 Python (2.7.6) (judge verdict: Time Limit Exceeded)

# -*- coding: utf-8 -*-
N = int(raw_input())
k = raw_input()
l = []
for i in range(0, len(k)):
	if i % 2 == 0:
		l.append(k[i])
d = {}
for c in range(0, len(l)):
	d[c+1] = l[c]
count = 0
for x,y in d.items():
	if (int(y), str(x)) in d.items():
		count +=1
print count / 2