Buggy python submission (verdict: Runtime Error):
import math,itertools,fractions,heapq,collections,bisect,sys,queue,copy

sys.setrecursionlimit(10**7)
inf=10**20
mod=10**9+7
dd=[(-1,0),(0,1),(1,0),(0,-1)]
ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def I(): return int(sys.stdin.readline())
def LS(): return sys.stdin.readline().split()
def S(): return input()

def main():
  s=S()
  i_f=0
  i_b=len(s)-1

  ans=0
  while True:
    if i_f==i_b:
      return ans

    if s[i_f]==s[i_b]:
      i_f+=1
      i_b-=1
    else:
      if s[i_f]=='x':
        i_f+=1
        ans+=1
      elif s[i_b]=='x':
        i_b-=1
        ans+=1
      else:
        return -1

# main()
print(main())


Fixed submission:
import math,itertools,fractions,heapq,collections,bisect,sys,queue,copy

sys.setrecursionlimit(10**7)
inf=10**20
mod=10**9+7
dd=[(-1,0),(0,1),(1,0),(0,-1)]
ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def I(): return int(sys.stdin.readline())
def LS(): return sys.stdin.readline().split()
def S(): return input()

def main():
  s=S()
  i_f=0
  i_b=len(s)-1

  ans=0
  while True:
    if i_f>=i_b:
      return ans

    # print(i_f,i_b)
    if s[i_f]==s[i_b]:
      i_f+=1
      i_b-=1
    else:
      if s[i_f]=='x':
        i_f+=1
        ans+=1
      elif s[i_b]=='x':
        i_b-=1
        ans+=1
      else:
        return -1

# main()
print(main())

Buggy python submission (verdict: Runtime Error):
import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,queue,copy

sys.setrecursionlimit(10**7)
inf=10**20
mod=10**9+7
dd=[(-1,0),(0,1),(1,0),(0,-1)]
ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def I(): return int(sys.stdin.readline())
def LS(): return sys.stdin.readline().split()
def S(): return input()

# a〜z -- START --
l=['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
# a〜z --- END ---

def main():
  s=S()

  _s=s.replace('x','')

  # 可能か
  if _s!=''.join([x for x in _s[::-1]]):
    return -1

  center=_s[len(_s)//2]
  to_center_count=0

  for x in _s[len(_s)//2:]:
    if x==center:
      to_center_count+=1

  count=0
  for i in range(len(s)):
    if s[i]==center:
      count+=1

    if count==to_center_count:
      ptr1=i
      break

  ptr2=ptr1
  if len(_s)%2==0:
    while True:
      ptr1-=1
      if s[ptr1]!='x':
        break

  ans=0
  while True:
    if ptr1==-1:
      return len(s)-ptr2+ans
    if ptr2==len(s):
      
      return ptr1+1+ans

    if s[ptr1]=='x':
      ptr1-=1
      if s[ptr2]=='x':
        ptr2+=1
      else:
        ans+=1
    elif s[ptr2]=='x':
      ptr2+=1
      if s[ptr1]=='x':
        ptr1-=1
      else:
        ans+=1
    else:
      ptr1-=1
      ptr2+=1

# main()
print(main())


Fixed submission:
import math,itertools,fractions,heapq,collections,bisect,sys,queue,copy

sys.setrecursionlimit(10**7)
inf=10**20
mod=10**9+7
dd=[(-1,0),(0,1),(1,0),(0,-1)]
ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def I(): return int(sys.stdin.readline())
def LS(): return sys.stdin.readline().split()
def S(): return input()

def main():
  s=S()
  i_f=0
  i_b=len(s)-1

  ans=0
  while True:
    if i_f>=i_b:
      return ans

    # print(i_f,i_b)
    if s[i_f]==s[i_b]:
      i_f+=1
      i_b-=1
    else:
      if s[i_f]=='x':
        i_f+=1
        ans+=1
      elif s[i_b]=='x':
        i_b-=1
        ans+=1
      else:
        return -1

# main()
print(main())