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 PyPy3 (2.4.0) (judge verdict: Wrong Answer)

N=int(input())
A=[int(i) for i in input().split()]
T=[2**i for i in range(1,35)]
table=[True]*N
A.sort()
ans=0
for t in T[::-1]:
  l,r=0,N-1
  while l<r:
    if A[l]+A[r]>t:
      r-=1
    elif A[l]+A[r]==t:
      if table[l] and table[r]:
        ans+=1
        table[l],table[r]=False,False
        l+=1
      r-=1
    elif A[l]+A[r]<t:
      l+=1
print(ans)