output_description: Print the number of pairs satisfying the condition.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      A_1 A_2 \dots A_N
attempt: num = int(input())
  a = input().split(" ")
  for i in range(num):
      a[i] = int(a[i])
  count = 0
  for i in range(num - 1):
      for j in range(i + 1, num):
          id = j - i
          x = abs(a[j] - a[i])
          if x == id:
              count += 1
  print(count)
  
problem_description: Statement
  
  You are the top spy of AtCoder Kingdom. To prevent the stolen secret from
  being handed to AlDebaran Kingdom, you have sneaked into the party where the
  transaction happens.
  
  There are N attendees in the party, and they are given attendee numbers from 1
  through N. The height of Attendee i is A_i.
  
  According to an examination beforehand, you know that a pair of attendees
  satisfying the condition below will make the transaction.
  
    * The absolute difference of their attendee numbers is equal to the sum of their heights.
  
  There are \frac{N(N-1)}{2} ways to choose two from the N attendees and make a
  pair. Among them, how many satisfy the condition above?
  
  P.S.: We cannot let you know the secret.
samples: [{"input": "6\n    2 3 3 1 3 1", "output": "3\n    \n\n  * A_1 + A_4 = 3, so the pair of Attendee 1 and 4 satisfy the condition.\n  * A_2 + A_6 = 4, so the pair of Attendee 2 and 6 satisfy the condition.\n  * A_4 + A_6 = 2, so the pair of Attendee 4 and 6 satisfy the condition.\n\nNo other pair satisfies the condition, so you should print 3.\n\n* * *"}, {"input": "6\n    5 2 4 2 8 8", "output": "0\n    \n\nNo pair satisfies the condition, so you should print 0.\n\n* * *"}, {"input": "32\n    3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5", "output": "22"}]