Buggy python submission (verdict: Time Limit Exceeded):
from collections import defaultdict
import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
A.sort(reverse=True)
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1
n = 1
while n <= 2 * A[0]:
    n *= 2
n //= 2
ans = 0
while n > 0:
    for a in A:
        if cnt[a] == 0:
            continue
        b = n - a
        if b > a:
            break
        c = 0
        if b == a and cnt[a] > 1:
            c = cnt[a] // 2
        elif b != a and cnt[b] > 0:
            c = min(cnt[a], cnt[b])
        cnt[a] -= c
        cnt[b] -= c
        ans += c
    n //= 2
print(ans)

Fixed submission:
from collections import defaultdict
import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
A.sort(reverse=True)
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1
n = 1
while n <= 2 * A[0]:
    n *= 2
n //= 2
ans = 0
while n > 0:
    for a in A:
        if cnt[a] == 0:
            continue
        b = n - a
        if b < 0:
            continue
        if b > a:
            break
        c = 0
        if b == a and cnt[a] > 1:
            c = cnt[a] // 2
        elif b != a and cnt[b] > 0:
            c = min(cnt[a], cnt[b])
        cnt[a] -= c
        cnt[b] -= c
        ans += c
    n //= 2
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import defaultdict
import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
A.sort(reverse=True)
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1
n = 1
while n <= 2 * A[0]:
    n *= 2
n //= 2
ans = 0
while n > 0:
    for a in A:
        if cnt[a] == 0:
            continue
        b = n - a
        if b > a:
            break
        c = 0
        if b == a and cnt[a] > 1:
            c = cnt[a] // 2
        elif b != a and cnt[b] > 0:
            c = min(cnt[a], cnt[b])
        cnt[a] -= c
        cnt[b] -= c
        ans += c
    n //= 2
print(ans)

Fixed submission:
from collections import defaultdict
import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
A.sort(reverse=True)
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1
n = 1
while n <= 2 * A[0]:
    n *= 2
n //= 2
ans = 0
while n > 0:
    for a in A:
        if cnt[a] == 0:
            continue
        b = n - a
        if b < 0:
            continue
        if b > a:
            break
        c = 0
        if b == a and cnt[a] > 1:
            c = cnt[a] // 2
        elif b != a and cnt[b] > 0:
            c = min(cnt[a], cnt[b])
        cnt[a] -= c
        cnt[b] -= c
        ans += c
    n //= 2
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import defaultdict
import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1
n = 1
m = max(A)
while n <= 2 * m:
    n *= 2
n //= 2
ans = 0
while n > 0:
    for a in A:
        if cnt[a] == 0:
            continue
        b = n - a
        c = 0
        if b == a and cnt[a] > 1:
            c = cnt[a] // 2
        elif b != a and cnt[b] > 0:
            c = min(cnt[a], cnt[b])
        cnt[a] -= c
        cnt[b] -= c
        ans += c
    n //= 2
print(ans)

Fixed submission:
from collections import defaultdict
import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
A.sort(reverse=True)
cnt = defaultdict(int)
for a in A:
    cnt[a] += 1
n = 1
while n <= 2 * A[0]:
    n *= 2
n //= 2
ans = 0
while n > 0:
    for a in A:
        if cnt[a] == 0:
            continue
        b = n - a
        if b < 0:
            continue
        if b > a:
            break
        c = 0
        if b == a and cnt[a] > 1:
            c = cnt[a] // 2
        elif b != a and cnt[b] > 0:
            c = min(cnt[a], cnt[b])
        cnt[a] -= c
        cnt[b] -= c
        ans += c
    n //= 2
print(ans)