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: import math
  N = int(input())
  l = list(map(int, input().split()))
  ld = sorted(l, reverse=True)
  
  k = 0
  two = []
  while 2**k < ld[0] + ld[1]:
      k+=1
      two.append(2**k)
  p = 0
  
  while len(ld) > 0:
      for m in range(1,len(ld)):
          if (ld[0] + ld[m]) in two:
              del ld[m]
              p += 1
              break
      if len(ld) > 0:
          del ld[0]
      if len(ld) = 0:
          break
  print(p)
  
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"}]