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 Ruby (2.3.3) (judge verdict: Accepted)

n = gets.to_i
a = gets.chomp.split(" ").map(&:to_i)

#b = Array.new(n+1).map{Array.new}
h = Hash.new{|k,v| k[v]=0}

a.each_with_index do |n, i|
  t = [n, i+1].sort!
  h[t] += 1
  #b[n] << i+1
  #b[i+1] << n
end
p h.to_a.select{|t| t[1]==2}.size


#b.select{|t| }