Buggy python submission (verdict: Time Limit Exceeded):
import collections
 
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  for j in range(31):
    if(2**j>i):
      k=2**j-i
      break
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Fixed submission:
import collections
import math
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  j=int(math.log2(i))+1
  k=2**j-i
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Buggy python submission (verdict: Runtime Error):
import collections
 
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  for j in range(30):
    if(2**j>i):
      k=2**j-i
      break
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Fixed submission:
import collections
import math
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  j=int(math.log2(i))+1
  k=2**j-i
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Buggy python submission (verdict: Runtime Error):
import collections
 
n=int(input())
a=list(map(int, input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  for j in range(20):
    if(2**j>i):
      k=2**j-i
      break
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Fixed submission:
import collections
import math
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  j=int(math.log2(i))+1
  k=2**j-i
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Buggy python submission (verdict: Runtime Error):
import collections
 
n=int(input())
a=list(map(int, input().split()))
a.sort()
a.reverse()
c=collections.Counter(a)
ans=0
for i in a:
  for j in range(20):
    if(2**j>i):
      k=2**j-i
      break
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

Fixed submission:
import collections
import math
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  j=int(math.log2(i))+1
  k=2**j-i
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

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

def dfs(v,visited):
  for u in m2[v]:
    if u in visited:
      continue
    visited.add(u)
    if(n2[u]==-1 or dfs(n2[u],visited)):
      n2[u]=v
      return True
  return False

n=int(input())
a=list(map(int,input().split()))
a.sort()
b=[]
kari=[]
ma=[]
m2=[]
n2=[]
vt=[]
vp=[]
visited=[]
ans=0
for i in range(n):
  kari=[]
  for j in range(20):
    for k in range(bisect.bisect_left(a,2**j-a[i]),bisect.bisect_right(a,2**j-a[i])):
      if(k==i):
        continue
      kari.append(k)
  ma.append(kari)
for i in range(n):
  kari=[]
  for j in ma[i]:
    kari.append(j+n)
  m2.append(kari)
for i in range(2*n):
  n2.append(-1)
print(sum(dfs(s,set())for s in range(n))//2)


Fixed submission:
import collections
import math
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  j=int(math.log2(i))+1
  k=2**j-i
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)

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

def dfs(v,visited):
  for u in m2[v]:
    if u in visited:
      continue
    visited.add(u)
    if(n2[u]==-1 or dfs(n2[u],visited)):
      n2[u]=v
      return True
  return False

n=int(input())
a=list(map(int,input().split()))
a.sort()
b=[]
kari=[]
ma=[]
m2=[]
n2=[]
vt=[]
vp=[]
visited=[]
ans=0
for i in range(n):
  kari=[]
  for j in range(20):
    for k in range(bisect.bisect_left(a,2**j-a[i]),bisect.bisect_right(a,2**j-a[i])):
      if(k==i):
        continue
      kari.append(k)
  ma.append(kari)
for i in range(n):
  kari=[]
  for j in range(n):
    if(j in ma[i]):
      kari.append(j+n)
  m2.append(kari)
for i in range(2*n):
  n2.append(-1)
print(sum(dfs(s,set())for s in range(n))//2)

Fixed submission:
import collections
import math
n=int(input())
a=list(map(int,input().split()))
a.sort(reverse=True)
c=collections.Counter(a)
ans=0
for i in a:
  j=int(math.log2(i))+1
  k=2**j-i
  if k in c.keys():
    if i==k and c[i]>1:
      c[i]-=2
      ans+=1
    elif i!=k and c[i]>0 and c[k]>0:
      c[i]-=1
      c[k]-=1
      ans+=1
print(ans)