Problem:
Score : 600 points

Problem 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.

Constraints

1 \leq N \leq 2\times 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

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.

Sample Input 1

3
1 2 3

Sample Output 1

1

We can form one pair whose sum of the written numbers is 4 by pairing the first and third balls.
Note that we cannot pair the second ball with itself.

Sample Input 2

5
3 11 14 5 13

Sample Output 2

2

Solution in Ruby (2.3.3) (judge verdict: Wrong Answer)

def div(n)
  x = 0
  while n>1
    x += 1
    n /= 2
  end
  list = [2]
  x.times do |i|
    list << list[i]*2
  end
  return list
end

size = gets.to_i
a_list = p gets.split.map(&:to_i).sort.reverse
res = 0
two = div(a_list.first)

a_list.each_with_index do |x, i|
  two.pop if two.last < x
  target = two.last - x

  (i+1).upto(size-1) do |y|
    if target == a_list[y]    
      res += 1
      a_list[y] = 0
      break
    end
  end
end

puts res