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 PyPy3 (2.4.0) (judge verdict: Time Limit Exceeded)

import itertools

n=int(input())
a=list(map(int,input().split()))
l=list(range(1,n+1))
tmpList= (list([x,y] for x,y in (itertools.combinations(l, 2)) if x<y))
ans=0
for item in tmpList:
    if a[item[0]-1]!=item[0] and a[item[1]-1]!=item[1] and a[item[1]-1]==item[0] and a[item[0]-1]==item[1]:
        ans+=1
print (ans)