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=list(map(int,input().split()))
al=list(set(a))
m=max(a).bit_length()+1
ans=0
used=set()
for i in range(m,0,-1):
  s=1<<i
  d={}
  for j in range(n):
    if j in used:continue
    if s//2<=a[j]<s:
      if s-a[j] in d:d[s-a[j]]+=1
      else:d[s-a[j]]=1
      used.add(j)
  for j in range(n):
    if j in used:continue
    if a[j] in d:
      if d[a[j]]:
        ans+=1
        d[a[j]]-=1
        used.add(j)
  if s//2 in d:ans+=d[s//2]*(d[s//2]-1)//2
print(ans)