output_description: Print the maximum possible number of pairs such that the sum of the integers
  written on each pair of balls is a power of 2.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      A_1 A_2 ... A_N
attempt: # AGC030 B - Powers of two
  import copy
  
  beki = []
  for i in range(1, 32):
      beki.append(2**i)  # bekiは２のべき乗の配列
  N = int(input())
  A = input().split()
  for i in range(N):
      A[i] = int(A[i])
  for i in reversed(beki):
      B = copy.deepcopy(A)
      for j in B:
          if i // 2 < j < i:
              if i - j in B:
                  A.remove(i - j)
                  A.remove(j)
          if j == i // 2:
              if B.count(j) >= 2:
                  A.remove(j)
                  A.remove(j)
  print((N - len(A)) // 2)
  
problem_description: Statement
  
  Takahashi has N balls with positive integers written on them. The integer
  written on the i-th ball is A_i. He would like to form some number of pairs
  such that the sum of the integers written on each pair of balls is a power of
  2. Note that a ball cannot belong to multiple pairs. Find the maximum possible
  number of pairs that can be formed.
  
  Here, a positive integer is said to be a power of 2 when it can be written as
  2^t using some non-negative integer t.
samples: [{"input": "3\n    1 2 3", "output": "1\n    \n\nWe can form one pair whose sum of the written numbers is 4 by pairing the\nfirst and third balls. Note that we cannot pair the second ball with itself.\n\n* * *"}, {"input": "5\n    3 11 14 5 13", "output": "2"}]