Problem:
Score : 400 points

Problem Statement

We have a string s consisting of lowercase English letters.
Snuke can perform the following operation repeatedly:

Insert a letter x to any position in s of his choice, including the beginning and end of s.

Snuke's objective is to turn s into a palindrome.
Determine whether the objective is achievable. If it is achievable, find the minimum number of operations required.

Notes

A palindrome is a string that reads the same forward and backward.
For example, a, aa, abba and abcba are palindromes, while ab, abab and abcda are not.

Constraints

1 \leq |s| \leq 10^5

s consists of lowercase English letters.

Input

Input is given from Standard Input in the following format:

s

Output

If the objective is achievable, print the number of operations required.
If it is not, print -1 instead.

Sample Input 1

xabxa

Sample Output 1

2

One solution is as follows (newly inserted x are shown in bold):

xabxa → xaxbxa → xaxbxax

Sample Input 2

ab

Sample Output 2

-1

No sequence of operations can turn s into a palindrome.

Sample Input 3

a

Sample Output 3

0

s is a palindrome already at the beginning.

Sample Input 4

oxxx

Sample Output 4

3

One solution is as follows:

oxxx → xoxxx → xxoxxx → xxxoxxx

Solution in Python (3.4.3) (judge verdict: Runtime Error)

def palindrome(s):
  for i in range(len(s) // 2):
    if s[i] != s[-i -1]:
      return False
  return True

def main():
  s = input()
  t = ''
  for ch in s:
    if ch != 'x':
      t += ch

  if (palindrome(t) == False):
    print(-1)
    return

  cnt = 0
  odd = len(t) & 1
  mid = len(t) // 2

  if odd == 1:  # ax[b]a

    for i in range(len(s)):
      if s[i] != 'x':
        cnt += 1
        if cnt == mid + 1:
          s1 = s[      : i]
          s2 = s[i + 1 :  ]
          break    

  else: # a[x]a

    for i in range(len(s)):
      if s[i] != 'x':
        cnt += 1
        if cnt == mid:
          s1 = s[  : i + 1]
        if cnt == mid + 1:
          s2 = s[i :  ]
          break

  st = ''
  for ch in s2:
    st = ch + st

  # print(s1, st)
  ans = 0
  i = 0
  j = 0
  while (i < len(s1) and j < len(st)):
    c1 = s1[i]
    c2 = st[j]
    if c1 == c2:
      i += 1
      j += 1
      continue

    ans += 1
    if c1 == 'x':
      i += 1
    else:
      j += 1

  ans += (len(s1) - i) + (len(st) - j)
  print(ans)






if __name__ == '__main__':
  main()