Buggy python submission (verdict: Time Limit Exceeded):
import sys

def solve():
    N = int(sys.stdin.readline().rstrip())
    A = list(map(int,sys.stdin.readline().rstrip().split()))
    A = sorted(A, reverse=True)
    maxA = A[0]
    cntA = [0 for i in range(maxA+1)]
    for a in A:
        cntA[a] += 1

    ans = 0
    ones = (1 << 31) - 1
    for a in A:
        if cntA[a] == 0:
            continue
        cntA[a] -= 1
        #print('a =',a)
        b = (1 << a.bit_length()) - a
        if cntA[b] > 0:
            cntA[b] -= 1
            ans += 1
    return ans

print(solve())

#  1  1  7   9
#     3  5   11
#        13
#     15
#  2  6  10
#     14
#  4  4  12
#
#  8


Fixed submission:
import sys
import collections

def solve():
    N = int(sys.stdin.readline().rstrip())
    A = list(map(int,sys.stdin.readline().rstrip().split()))
    A = sorted(A, reverse=True)
    maxA = A[0]
    cntA = collections.Counter(A)

    ans = 0
    for a in A:
        if cntA[a] == 0:
            continue
        cntA[a] -= 1
        #print('a =',a)
        b = (1 << a.bit_length()) - a
        if cntA[b] > 0:
            cntA[b] -= 1
            ans += 1
    return ans

print(solve())

#  1  1  7   9
#     3  5   11
#        13
#     15
#  2  6  10
#     14
#  4  4  12
#
#  8

Buggy python submission (verdict: Runtime Error):
import sys

def solve():
    N = int(sys.stdin.readline().rstrip())
    A = list(map(int,sys.stdin.readline().rstrip().split()))
    A = sorted(A, reverse=True)
    maxA = A[0]
    cntA = [0] * (maxA+1)
    for a in A:
        cntA[a] += 1

    ans = 0
    twos = 1 << 32
    for a in A:
        if a <= 2:
            return ans
        if cntA[a] == 0:
            continue
        cntA[a] -= 1
        #print('a =',a)
        while True:
            b = twos - a
            #print('b =',b)
            if b <= a:
                break
            twos //= 2
            if twos == 1:
                return ans
        if cntA[b] > 0:
            cntA[b] -= 1
            ans += 1
    return ans

print(solve())


Fixed submission:
import sys
import collections

def solve():
    N = int(sys.stdin.readline().rstrip())
    A = list(map(int,sys.stdin.readline().rstrip().split()))
    A = sorted(A, reverse=True)
    maxA = A[0]
    cntA = collections.Counter(A)

    ans = 0
    for a in A:
        if cntA[a] == 0:
            continue
        cntA[a] -= 1
        #print('a =',a)
        b = (1 << a.bit_length()) - a
        if cntA[b] > 0:
            cntA[b] -= 1
            ans += 1
    return ans

print(solve())

#  1  1  7   9
#     3  5   11
#        13
#     15
#  2  6  10
#     14
#  4  4  12
#
#  8

Buggy python submission (verdict: Time Limit Exceeded):
import sys

def solve():
    N = int(sys.stdin.readline().rstrip())
    A = list(map(int,sys.stdin.readline().rstrip().split()))
    A = sorted(A, reverse=True)
    maxA = A[0]
    cntA = [0 for i in range(maxA+1)]
    for a in A:
        cntA[a] += 1

    ans = 0
    ones = (1 << 31) - 1
    for a in A:
        if cntA[a] == 0:
            continue
        cntA[a] -= 1
        #print('a =',a)
        while True:
            b = (ones ^ a) + 1
            #print('b =',b)
            if b <= a:
                break
            ones >>= 1
            if ones == 0:
                return ans
        if cntA[b] > 0:
            cntA[b] -= 1
            ans += 1
    return ans

print(solve())


Fixed submission:
import sys
import collections

def solve():
    N = int(sys.stdin.readline().rstrip())
    A = list(map(int,sys.stdin.readline().rstrip().split()))
    A = sorted(A, reverse=True)
    maxA = A[0]
    cntA = collections.Counter(A)

    ans = 0
    for a in A:
        if cntA[a] == 0:
            continue
        cntA[a] -= 1
        #print('a =',a)
        b = (1 << a.bit_length()) - a
        if cntA[b] > 0:
            cntA[b] -= 1
            ans += 1
    return ans

print(solve())

#  1  1  7   9
#     3  5   11
#        13
#     15
#  2  6  10
#     14
#  4  4  12
#
#  8