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 Python (3.4.3) (judge verdict: Time Limit Exceeded)

N = int(input())
A_arr = list(map(int, input().split()))

A_arr = sorted(A_arr, reverse=True)
two_arr = []
for i in range(1,31):
    two_arr.append(2**i)

count = 0
while len(A_arr) != 0:
    v_max = A_arr.pop(0)
    v_pair = 2**v_max.bit_length() - v_max
    if v_pair in A_arr:
        A_arr.remove(v_pair)
        count += 1
    else:
        pass

print(count)