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

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

alist.sort()
#print(alist)

flist=[False]*N
answer=0
for i in reversed(range(N)):
  if flist[i]:
    continue
  
  p1=alist[i]
  p2=2**math.ceil(math.log(p1+1,2))-p1
  #print(p1,p2)
  
  index1 = bisect.bisect_left(alist,p2,0,i+1)  
  index2 = bisect.bisect_right(alist,p2,index1,i+1)  
  #print(index1,index2)
  for j in range(index1,index2):
    if j != i and not flist[j]:
      flist[i]=True
      flist[j]=True
      answer+=1
      break
  
#print(flist)    
print(answer)


Fixed submission:
import bisect
N=int(input())
alist=list(map(int,input().split()))
alist.sort(reverse=True)

blist=[]
for a in alist:
  a2=a
  digit_a2=0
  while(a2>0):
    a2>>=1
    digit_a2+=1
  b=2**(digit_a2)-a
  blist.append(b)
#print(alist)
#print(blist)

dic_a={}
for a in alist:
  if a in dic_a:
    dic_a[a]+=1
  else:
    dic_a[a]=1

answer=0
for i in range(N):
  a=alist[i]
  b=blist[i]
  if a==b:
    if dic_a[a]>1:
      dic_a[a]-=2
      answer+=1
      #print(a,a)
  else:
    if dic_a[a]>0 and b in dic_a and dic_a[b]>0:
      dic_a[a]-=1
      dic_a[b]-=1
      answer+=1
      #print(a,b)
    
print(answer)

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

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

alist.sort()
#print(alist)

flist=[False]*N
answer=0
for i in reversed(range(N)):
  if flist[i]:
    continue
  
  p1=alist[i]
  p2=2**math.ceil(math.log(p1+1,2))-p1
  #print(p1,p2)
  
  index1 = bisect.bisect_left(alist,p2)  
  index2 = bisect.bisect_right(alist,p2)  
  #print(index1,index2)
  for j in range(index1,index2):
    if j != i and not flist[j]:
      flist[i]=True
      flist[j]=True
      answer+=1
      break
  
#print(flist)    
print(answer)


Fixed submission:
import bisect
N=int(input())
alist=list(map(int,input().split()))
alist.sort(reverse=True)

blist=[]
for a in alist:
  a2=a
  digit_a2=0
  while(a2>0):
    a2>>=1
    digit_a2+=1
  b=2**(digit_a2)-a
  blist.append(b)
#print(alist)
#print(blist)

dic_a={}
for a in alist:
  if a in dic_a:
    dic_a[a]+=1
  else:
    dic_a[a]=1

answer=0
for i in range(N):
  a=alist[i]
  b=blist[i]
  if a==b:
    if dic_a[a]>1:
      dic_a[a]-=2
      answer+=1
      #print(a,a)
  else:
    if dic_a[a]>0 and b in dic_a and dic_a[b]>0:
      dic_a[a]-=1
      dic_a[b]-=1
      answer+=1
      #print(a,b)
    
print(answer)

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

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

alist.sort()
#print(alist)

flist=[False]*N
answer=0
for i in reversed(range(N)):
  if flist[i]:
    continue
  
  p1=alist[i]
  p2=2**math.ceil(math.log(p1+1,2))-p1
  #print(p1,p2)
  
  index1 = bisect.bisect_left(alist,p2)  
  index2 = bisect.bisect_right(alist,p2)  
  #print(index1,index2)
  for j in range(index1,index2):
    if j != i and not flist[j]:
      flist[i]=True
      flist[j]=True
      answer+=1
      break
  
#print(flist)    
print(answer)

Fixed submission:
import bisect
N=int(input())
alist=list(map(int,input().split()))
alist.sort(reverse=True)

blist=[]
for a in alist:
  a2=a
  digit_a2=0
  while(a2>0):
    a2>>=1
    digit_a2+=1
  b=2**(digit_a2)-a
  blist.append(b)
#print(alist)
#print(blist)

dic_a={}
for a in alist:
  if a in dic_a:
    dic_a[a]+=1
  else:
    dic_a[a]=1

answer=0
for i in range(N):
  a=alist[i]
  b=blist[i]
  if a==b:
    if dic_a[a]>1:
      dic_a[a]-=2
      answer+=1
      #print(a,a)
  else:
    if dic_a[a]>0 and b in dic_a and dic_a[b]>0:
      dic_a[a]-=1
      dic_a[b]-=1
      answer+=1
      #print(a,b)
    
print(answer)

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

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

alist.sort()
#print(alist)

answer=0
while(True):
  if len(alist) <= 1:
    break
  
  p1=alist.pop()
  p2=2**math.ceil(math.log(p1+1,2))-p1
  
  index1 = bisect.bisect_left(alist,p2)  
  if index1 != bisect.bisect_right(alist,p2):
    alist.pop(index1)
    answer+=1
    
print(answer)

Fixed submission:
import bisect
N=int(input())
alist=list(map(int,input().split()))
alist.sort(reverse=True)

blist=[]
for a in alist:
  a2=a
  digit_a2=0
  while(a2>0):
    a2>>=1
    digit_a2+=1
  b=2**(digit_a2)-a
  blist.append(b)
#print(alist)
#print(blist)

dic_a={}
for a in alist:
  if a in dic_a:
    dic_a[a]+=1
  else:
    dic_a[a]=1

answer=0
for i in range(N):
  a=alist[i]
  b=blist[i]
  if a==b:
    if dic_a[a]>1:
      dic_a[a]-=2
      answer+=1
      #print(a,a)
  else:
    if dic_a[a]>0 and b in dic_a and dic_a[b]>0:
      dic_a[a]-=1
      dic_a[b]-=1
      answer+=1
      #print(a,b)
    
print(answer)

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

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

alist.sort()
#print(alist)

answer=0
while(True):
  if len(alist) <= 1:
    break
  
  p1=alist.pop()
  p2=2**math.ceil(math.log(p1+1,2))-p1
  
  index1 = bisect.bisect_left(alist,p2)  
  if index1 != bisect.bisect_right(alist,p2):
    alist.pop(index1)
    answer+=1
    
print(answer)

Fixed submission:
import bisect
N=int(input())
alist=list(map(int,input().split()))
alist.sort(reverse=True)

blist=[]
for a in alist:
  a2=a
  digit_a2=0
  while(a2>0):
    a2>>=1
    digit_a2+=1
  b=2**(digit_a2)-a
  blist.append(b)
#print(alist)
#print(blist)

dic_a={}
for a in alist:
  if a in dic_a:
    dic_a[a]+=1
  else:
    dic_a[a]=1

answer=0
for i in range(N):
  a=alist[i]
  b=blist[i]
  if a==b:
    if dic_a[a]>1:
      dic_a[a]-=2
      answer+=1
      #print(a,a)
  else:
    if dic_a[a]>0 and b in dic_a and dic_a[b]>0:
      dic_a[a]-=1
      dic_a[b]-=1
      answer+=1
      #print(a,b)
    
print(answer)