Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque
S = deque(input())

ans = 0
while len(S):
    if S[0] == S[-1]:
        if len(S) > 1:
            S.popleft()
            S.pop()
        else:
            S.pop()
    else:
        if S[0] != "x" and S[1] != "x":
            print(-1)
            exit()
        elif S[0] != "x":
            S.appendleft("x")
            ans += 1
        elif S[1] != "x":
            S.append("x")
            ans += 1
print(ans)

Fixed submission:
import sys

S = sys.stdin.readline().rstrip()
not_x = []

for i, s in enumerate(S):
    if s != "x":
        not_x.append(s)

not_x_nums = len(not_x)
mid = not_x_nums // 2

if not_x_nums % 2 == 1:
    left = not_x[:mid]
    right = not_x[mid + 1:][::-1]
else:
    left = not_x[:mid]
    right = not_x[mid:][::-1]

if left == right:
    ans = 0
    left = []
    right = []
    tmp = 0
    cnt = 0
    if not_x_nums % 2 == 1:
        mid += 1
    for s in S:
        if s != "x":
            left.append(tmp)
            tmp = 0
            cnt += 1
        else:
            tmp += 1
        if cnt == mid:
            break
    tmp = 0
    cnt = 0
    for s in S[::-1]:
        if s != "x":
            right.append(tmp)
            tmp = 0
            cnt += 1
        else:
            tmp += 1
        if cnt == mid:
            break
    for l, r in zip(left, right):
        ans += abs(l - r)
    print(ans)
else:
    print(-1)

Buggy python submission (verdict: Runtime Error):
import sys

S = sys.stdin.readline().rstrip()
N = len(S)
not_x = []
x_cnt_idx = [0] * N
not_x_cnt_idx = [0] * N

for i, s in enumerate(S):
    if s != "x":
        not_x.append(s)
        not_x_cnt_idx[i] = not_x_cnt_idx[i - 1] + 1 if i > 0 else 1
    else:
        x_cnt_idx[i] = x_cnt_idx[i - 1] + 1 if i > 0 else 1

not_x_cnt = len(not_x)
x_cnt = N - not_x_cnt

if not_x_cnt == 0:
    ans = 0
    print(ans)
    exit()
elif not_x_cnt == 1:
    if N == 1:
        ans = 0
    else:
        x_left_cnt = x_cnt_idx[not_x_cnt_idx.index(1)]
        ans = abs(x_left_cnt - (x_cnt - x_left_cnt))
    print(ans)
    exit()

if not_x_cnt % 2 == 1:
    left = not_x[:not_x_cnt // 2]
    right = not_x[not_x_cnt // 2 + 1:][::-1]
else:
    left = not_x[:not_x_cnt // 2]
    right = not_x[not_x_cnt // 2:][::-1]

if left == right:
    if x_cnt == 0:
        ans = 0
    else:
        x_left_cnt = x_cnt_idx[not_x_cnt_idx.index(not_x_cnt // 2)]
        ans = abs(x_left_cnt - (x_cnt - x_left_cnt))
    print(ans)
else:
    print(-1)

Fixed submission:
import sys

S = sys.stdin.readline().rstrip()
not_x = []

for i, s in enumerate(S):
    if s != "x":
        not_x.append(s)

not_x_nums = len(not_x)
mid = not_x_nums // 2

if not_x_nums % 2 == 1:
    left = not_x[:mid]
    right = not_x[mid + 1:][::-1]
else:
    left = not_x[:mid]
    right = not_x[mid:][::-1]

if left == right:
    ans = 0
    left = []
    right = []
    tmp = 0
    cnt = 0
    if not_x_nums % 2 == 1:
        mid += 1
    for s in S:
        if s != "x":
            left.append(tmp)
            tmp = 0
            cnt += 1
        else:
            tmp += 1
        if cnt == mid:
            break
    tmp = 0
    cnt = 0
    for s in S[::-1]:
        if s != "x":
            right.append(tmp)
            tmp = 0
            cnt += 1
        else:
            tmp += 1
        if cnt == mid:
            break
    for l, r in zip(left, right):
        ans += abs(l - r)
    print(ans)
else:
    print(-1)