Buggy python submission (verdict: Runtime Error):
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for i in reversed(range(n)):
    if memo[a[i]] == 0:
        continue
    if memo[1 << (len(bin(a[i])) - 2) - a[i]] == 0:
        continue
    memo[a[i]] -= 1
    memo[1 << (len(bin(a[i])) - 2) - a[i]] -= 1
    count += 1

print(count)

Fixed submission:
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for i in reversed(range(n)):
    if memo[a[i]] == 0:
        continue
    if (memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 0 and (1 << (len(bin(a[i])) - 2)) - a[i] != a[i]) or memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 1:
        memo[a[i]] -= 1
        memo[(1 << (len(bin(a[i])) - 2)) - a[i]] -= 1
        count += 1

print(count)

Buggy python submission (verdict: Time Limit Exceeded):
import bisect
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for t in reversed(range(1, 31)):
    for i in reversed(range(n)):
        if memo[a[i]] == 0:
            continue
        front = bisect.bisect_left(a, (1 << t) - a[i])
        back = bisect.bisect_right(a, (1 << t) - a[i])
        if front == back:
            continue
        if a[front] == a[i] and memo[a[i]] <= 1:
            continue
        if memo[a[front]] >= 1:
            memo[a[front]] -= 1
            memo[a[i]] -= 1
            count += 1

print(count)

Fixed submission:
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for i in reversed(range(n)):
    if memo[a[i]] == 0:
        continue
    if (memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 0 and (1 << (len(bin(a[i])) - 2)) - a[i] != a[i]) or memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 1:
        memo[a[i]] -= 1
        memo[(1 << (len(bin(a[i])) - 2)) - a[i]] -= 1
        count += 1

print(count)

Buggy python submission (verdict: Time Limit Exceeded):
import bisect
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for t in reversed(range(1, 31)):
    for i in reversed(range(n)):
        if memo[a[i]]== 0:
            continue
        front = bisect.bisect_left(a, (1 << t) - a[i])
        back = bisect.bisect_right(a, (1 << t) - a[i])
        if front == back:
            continue
        if a[front] == a[i] and memo[a[i]] <= 1:
            continue
        if memo[a[front]] >= 1:
            memo[a[front]] -= 1
            memo[a[i]] -= 1
            count += 1

print(count)

Fixed submission:
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for i in reversed(range(n)):
    if memo[a[i]] == 0:
        continue
    if (memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 0 and (1 << (len(bin(a[i])) - 2)) - a[i] != a[i]) or memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 1:
        memo[a[i]] -= 1
        memo[(1 << (len(bin(a[i])) - 2)) - a[i]] -= 1
        count += 1

print(count)

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

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = [0 for i in range(n)]

count = 0
for t in reversed(range(1, 31)):
    for i in reversed(range(n)):
        if memo[i] == 1:
            continue
        front = bisect.bisect_left(a, (1 << t) - a[i])
        back = bisect.bisect_right(a, (1 << t) - a[i])
        while front != back:
            if i <= front:
                break
            if memo[front] == 0:
                memo[i] = 1
                memo[front] = 1
                count += 1
                break
            front += 1

print(count)

Fixed submission:
from collections import defaultdict

n = int(input())
a = list(map(int, input().split()))

a.sort()
memo = defaultdict(int)
for i in range(n):
    memo[a[i]] += 1

count = 0
for i in reversed(range(n)):
    if memo[a[i]] == 0:
        continue
    if (memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 0 and (1 << (len(bin(a[i])) - 2)) - a[i] != a[i]) or memo[(1 << (len(bin(a[i])) - 2)) - a[i]] > 1:
        memo[a[i]] -= 1
        memo[(1 << (len(bin(a[i])) - 2)) - a[i]] -= 1
        count += 1

print(count)