Buggy python submission (verdict: Runtime Error):
# coding:utf-8

import sys
from bisect import bisect_left, bisect_right
INF = float('inf')
MOD = 10 ** 9 + 7
dy, dx = (0, -1, 0, 1), (1, 0, -1, 0)

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def II(): return int(sys.stdin.readline())
def SI(): return input()


def main():
    n = II()
    A = LI()
    A.sort()

    res = 0
    used = [0] * n
    for p in range(1, max(A).bit_length() + 1)[::-1]:
        t = 2 ** p
        for i in range(n)[::-1]:
            a = A[i]
            if used[i]:
                continue
            if a >= t:
                break
            bi = bisect_left(A, t - a)
            if A[bi] == t - a:
                if i != bi and not used[bi]:
                    res += 1
                    used[i] = 1
                    used[bi] = 1

    print(res)


if __name__ == '__main__':
    main()


Fixed submission:
# coding:utf-8

import sys
from collections import defaultdict

INF = float('inf')
MOD = 10 ** 9 + 7
dy, dx = (0, -1, 0, 1), (1, 0, -1, 0)

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def II(): return int(sys.stdin.readline())
def SI(): return input()


def main():
    n = II()
    A = LI()

    MA = defaultdict(int)
    # 目標値をO(1)で見つけるために辞書に格納
    for a in A:
        MA[a] += 1

    key_n = list(MA.keys())
    key_n.sort()
    key_n.reverse()

    res = 0
    p = 30
    for x in key_n:
        # すでにペアを作るために使用してしまっている時はパス
        if not MA[x]:
            continue

        # 残っているボールの中で一番大きな値xを選ぶと，そのボールを使って表せる2べきの値x + yは1つに定まる
        # x + 1 <= x + y < 2x
        # 2べきの比は2であるのに対して，x + 1と2xの比は2未満だから
        while 2 * x < 2 ** p:
            p -= 1

        if x + 1 <= 2 ** p:
            y = 2 ** p - x
            if not MA[y]:
                continue

            if x == y:
                _add = MA[x] // 2
            else:
                _add = min(MA[x], MA[y])

            res += _add
            MA[x] -= _add
            MA[y] -= _add

    print(res)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
# coding:utf-8

import bisect

INF = float('inf')
MOD = 10 ** 9 + 7


def inpl(): return list(map(int, input().split()))


def solve(num_cnt: list) -> int:
    num_cnt.sort()
    t = 2 ** (len(bin(num_cnt[-1])) - 2)
    used = [False] * len(num_cnt)
    cnt = 0
    for i in range(len(num_cnt))[::-1]:
        if used[i]:
            continue
        n = num_cnt[i]
        used[i] = True
        while 2 * n < t:
            t //= 2
        j = bisect.bisect_left(num_cnt, t - n)
        # print(n, t, j)
        if t - n == num_cnt[j] and n != t - n:
            cnt += 1
            used[j] = True

    return cnt


N = int(input())
A = inpl()

cnt_e, cnt_o = [], []
for a in A:
    if a % 2:
        cnt_o.append(a)
    else:
        cnt_e.append(a)

print(solve(cnt_e) + solve(cnt_o))


Fixed submission:
# coding:utf-8

import sys
from collections import defaultdict

INF = float('inf')
MOD = 10 ** 9 + 7
dy, dx = (0, -1, 0, 1), (1, 0, -1, 0)

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x) - 1 for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def II(): return int(sys.stdin.readline())
def SI(): return input()


def main():
    n = II()
    A = LI()

    MA = defaultdict(int)
    # 目標値をO(1)で見つけるために辞書に格納
    for a in A:
        MA[a] += 1

    key_n = list(MA.keys())
    key_n.sort()
    key_n.reverse()

    res = 0
    p = 30
    for x in key_n:
        # すでにペアを作るために使用してしまっている時はパス
        if not MA[x]:
            continue

        # 残っているボールの中で一番大きな値xを選ぶと，そのボールを使って表せる2べきの値x + yは1つに定まる
        # x + 1 <= x + y < 2x
        # 2べきの比は2であるのに対して，x + 1と2xの比は2未満だから
        while 2 * x < 2 ** p:
            p -= 1

        if x + 1 <= 2 ** p:
            y = 2 ** p - x
            if not MA[y]:
                continue

            if x == y:
                _add = MA[x] // 2
            else:
                _add = min(MA[x], MA[y])

            res += _add
            MA[x] -= _add
            MA[y] -= _add

    print(res)


if __name__ == '__main__':
    main()