Buggy python submission (verdict: Runtime Error):
from math import *
 
n=int(input())
arr=list(map(int,input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    if(dict1[arr[i]]>0):
        val2=int(ceil(log(arr[i],2)))
        xd=2**val2
        if(xd!=2*arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
    #print(dict1)
print(ans)

Fixed submission:
from math import *
 
n=int(raw_input())
arr=list(map(int,raw_input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    if(dict1[arr[i]]>0):
        xd=2**(len(bin(arr[i]))-2)
        if(xd-arr[i]!=arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
print(ans)

Buggy python submission (verdict: Runtime Error):
from math import *
 
n=int(raw_input())
arr=list(map(int,raw_input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    val2=int(ceil(log(arr[i],2)))
        xd=2**val2
        if(xd!=2*arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
                    break
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
                break
print(ans)

Fixed submission:
from math import *
 
n=int(raw_input())
arr=list(map(int,raw_input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    if(dict1[arr[i]]>0):
        xd=2**(len(bin(arr[i]))-2)
        if(xd-arr[i]!=arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
print(ans)

Buggy python submission (verdict: Runtime Error):
from math import *
 
n=int(input())
arr=list(map(int,input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    val2=ceil(log2(arr[i]))
    for j in range(33,val2-1,-1):
        xd=2**j
        if(xd!=2*arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
                    break
        
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
                break
print(ans)

Fixed submission:
from math import *
 
n=int(raw_input())
arr=list(map(int,raw_input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    if(dict1[arr[i]]>0):
        xd=2**(len(bin(arr[i]))-2)
        if(xd-arr[i]!=arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
print(ans)

Buggy python submission (verdict: Runtime Error):
from math import *

n=int(raw_input())
arr=list(map(int,raw_input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    val2=ceil(log(arr[i]))
    for j in range(33,val2-1,-1):
        xd=2**j
        if(xd!=2*arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    break
        
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=1
                break
    dict1[arr[i]]-=1
print(ans)





Fixed submission:
from math import *
 
n=int(raw_input())
arr=list(map(int,raw_input().split()))
dict1={}
for i in range(n):
    val=arr[i]
    try:
        dict1[val]+=1
    except:
        KeyError
        dict1[val]=1
#print(dict1)
ans=0
arr.sort(reverse=True)
for i in range(n):
    if(dict1[arr[i]]>0):
        xd=2**(len(bin(arr[i]))-2)
        if(xd-arr[i]!=arr[i]):
            try:
                if(dict1[xd-arr[i]]>0):
                    ans+=1
                    dict1[xd-arr[i]]-=1
                    dict1[arr[i]]-=1
            except:
                KeyError
        else:
            if(dict1[arr[i]]>1):
                ans+=1
                dict1[arr[i]]-=2
print(ans)