Buggy python submission (verdict: Runtime Error):
from collections import deque

s = input()

ns = ""

for s_i in s:
    if s_i != "x":
        ns += s_i

if ns != ns[::-1]:
    print(-1)
    exit()

if len(ns) % 2 == 1:
    cnt = -1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2:
            note = i
            break
    left = deque(s[:note][::-1] + "@")
    right = deque(s[note+1:] + "@")
else:
    cnt = -1
    f = 1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2 - 1 and f:
            l = i
            f = 0
        if cnt == len(ns)//2:
            r = i
            break
    left = deque(s[:l][::-1] + "@")
    right = deque(s[r+1:] + "@")

ans = 0

while left or right:
    if left[0] == right[0]:
        left.popleft()
        right.popleft()
    elif left[0] == "x":
        left.popleft()
        ans += 1
    elif right[0] == "x":
        right.popleft()
        ans += 1

print(ans)

Fixed submission:
from collections import deque

s = input()

ns = ""

for s_i in s:
    if s_i != "x":
        ns += s_i

if ns != ns[::-1]:
    print(-1)
    exit()

if ns == "":
    print(0)
    exit()

if len(ns) % 2 == 1:
    cnt = -1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2:
            note = i
            break
    left = deque(s[:note][::-1] + "@")
    right = deque(s[note+1:] + "@")
else:
    cnt = -1
    f = 1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2 - 1 and f:
            l = i
            f = 0
        if cnt == len(ns)//2:
            r = i
            break
    left = deque(s[:l][::-1] + "@")
    right = deque(s[r+1:] + "@")

ans = 0

while left or right:
    if left[0] == right[0]:
        left.popleft()
        right.popleft()
    elif left[0] == "x":
        left.popleft()
        ans += 1
    elif right[0] == "x":
        right.popleft()
        ans += 1

print(ans)

Buggy python submission (verdict: Runtime Error):
from collections import deque

s = input()

ns = ""

for s_i in s:
    if s_i != "x":
        ns += s_i

if ns != ns[::-1]:
    print(-1)
    exit()

if len(ns) % 2 == 1:
    cnt = -1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2:
            note = i
            break
    left = deque(s[:note][::-1] + "@")
    right = deque(s[note+1:] + "@")
else:
    cnt = -1
    f = 1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2 - 1 and f:
            l = i
            f = 0
        if cnt == len(ns)//2:
            r = i
            break
    left = deque(s[:l][::-1] + "@")
    right = deque(s[r+1:] + "@")

ans = 0

while left or right:
    print(left,right)
    if left[0] == right[0]:
        left.popleft()
        right.popleft()
    elif left[0] == "x":
        left.popleft()
        ans += 1
    elif right[0] == "x":
        right.popleft()
        ans += 1

print(ans)

Fixed submission:
from collections import deque

s = input()

ns = ""

for s_i in s:
    if s_i != "x":
        ns += s_i

if ns != ns[::-1]:
    print(-1)
    exit()

if ns == "":
    print(0)
    exit()

if len(ns) % 2 == 1:
    cnt = -1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2:
            note = i
            break
    left = deque(s[:note][::-1] + "@")
    right = deque(s[note+1:] + "@")
else:
    cnt = -1
    f = 1
    for i,s_i in enumerate(s):
        if s_i != "x":
            cnt += 1
        if cnt == len(ns)//2 - 1 and f:
            l = i
            f = 0
        if cnt == len(ns)//2:
            r = i
            break
    left = deque(s[:l][::-1] + "@")
    right = deque(s[r+1:] + "@")

ans = 0

while left or right:
    if left[0] == right[0]:
        left.popleft()
        right.popleft()
    elif left[0] == "x":
        left.popleft()
        ans += 1
    elif right[0] == "x":
        right.popleft()
        ans += 1

print(ans)