Score : 200 points
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.
Calculate the number of the friendly pairs.
The input is given from Standard Input in the following format:
N a_1 a_2 ... a_N
Print the number of the friendly pairs.
4 2 1 4 3
2
There are two friendly pairs: (1,2) and (3,4).
3 2 3 1
0
There are no friendly pairs.
5 5 5 5 5 1
1
There is one friendly pair: (1,5).