Buggy python submission (verdict: Time Limit Exceeded):
n = int(input())
A = list(map(int,input().split()))
A.append(-float("inf"))
n = len(A)

A.sort()
s = 2
while s <= A[-1]+A[-2]:
    s *= 2
s //= 2

U = [0] * n

ans = 0
while s > 0:
    ll = 0
    rr = n-1
    while rr >= 1 and rr-ll >= 2:
        # print("rr",rr,"ll",ll)
        t = s - A[rr]
        if t>s:
            break
        # print("s",s,"t",t)
        l = ll
        r = rr-1
        m = (r+l)//2
        while r-l > 1:
            # print("r",r,"l",l,"m",m)
            if A[m] >= t:
                r = m
            else:
                l = m
            m = (r+l)//2
        if A[l+1] == t:
            ans += 1
            U[l+1] = 1
            U[rr] = 1
            # print(ans)
            A[l+1] = -float("inf")
        ll = l
        rr -= 1
        # print(A)
    s //= 2

    B = []
    for i in range(len(A)):
        if U[i] == 0:
            B.append(A[i])
    A = B
    n = len(A)
    U = [0] * n
print(ans)


Fixed submission:
n = int(input())
A = list(map(int,input().split()))
A.append(-float("inf"))
n = len(A)

A.sort()
s = 2
while s <= A[-1]+A[-2]:
    s *= 2
s //= 2

U = [0] * n

ans = 0
while s > 0:
    ll = 0
    rr = n-1
    while rr >= 1 and s <= A[rr]:
        rr -= 1
    while rr >= 1 and rr-ll >= 2:
        # print("rr",rr,"ll",ll)
        t = s - A[rr]

        # print("s",s,"t",t)
        l = ll
        r = rr-1
        m = (r+l)//2
        while r-l > 1:
            # print("r",r,"l",l,"m",m)
            if A[m] >= t:
                r = m
            else:
                l = m
            m = (r+l)//2
        if A[l+1] == t:
            ans += 1
            U[l+1] = 1
            U[rr] = 1
            # print(ans)
            A[l+1] = -float("inf")
        ll = l
        rr -= 1
        # print(A)
    s //= 2

    B = []
    for i in range(len(A)):
        if U[i] == 0:
            B.append(A[i])
    A = B
    n = len(A)
    U = [0] * n
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
n = int(input())
A = list(map(int,input().split()))
A.append(-float("inf"))
n = len(A)

A.sort()
s = 2
while s <= A[-1]+A[-2]:
    s *= 2
s //= 2

U = [0] * n

ans = 0
while s > 0:
    ll = 0
    rr = n-1
    while rr >= 1 and rr-ll >= 2:
        # print("rr",rr,"ll",ll)
        t = s - A[rr]
        # print("s",s,"t",t)
        l = ll
        r = rr-1
        m = (r+l)//2
        while r-l > 1:
            # print("r",r,"l",l,"m",m)
            if A[m] >= t:
                r = m
            else:
                l = m
            m = (r+l)//2
        if A[l+1] == t:
            ans += 1
            U[l+1] = 1
            U[rr] = 1
            # print(ans)
            A[l+1] = -float("inf")
        ll = l
        rr -= 1
        # print(A)
    s //= 2

    B = []
    for i in range(len(A)):
        if U[i] == 0:
            B.append(A[i])
    A = B
    n = len(A)
    U = [0] * n
print(ans)


Fixed submission:
n = int(input())
A = list(map(int,input().split()))
A.append(-float("inf"))
n = len(A)

A.sort()
s = 2
while s <= A[-1]+A[-2]:
    s *= 2
s //= 2

U = [0] * n

ans = 0
while s > 0:
    ll = 0
    rr = n-1
    while rr >= 1 and s <= A[rr]:
        rr -= 1
    while rr >= 1 and rr-ll >= 2:
        # print("rr",rr,"ll",ll)
        t = s - A[rr]

        # print("s",s,"t",t)
        l = ll
        r = rr-1
        m = (r+l)//2
        while r-l > 1:
            # print("r",r,"l",l,"m",m)
            if A[m] >= t:
                r = m
            else:
                l = m
            m = (r+l)//2
        if A[l+1] == t:
            ans += 1
            U[l+1] = 1
            U[rr] = 1
            # print(ans)
            A[l+1] = -float("inf")
        ll = l
        rr -= 1
        # print(A)
    s //= 2

    B = []
    for i in range(len(A)):
        if U[i] == 0:
            B.append(A[i])
    A = B
    n = len(A)
    U = [0] * n
print(ans)