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 PyPy3 (2.4.0) (judge verdict: Wrong Answer)

from collections import Counter

s = input()
snotx = "".join([ch for ch in s if ch != 'x'])

if s == s[::-1]:
    print(0)
    exit()

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

# 回数を合わせる
# print(s)
# print(snotx)

# xのrun-length
rl = []
i = 0
while i < len(s): 
    j = i
    while j < len(s) and s[i] == s[j]:
        j += 1
    # print(s[i], i, j)
    if s[i] == 'x':
        rl.append(j - i)
    i = j
# print(rl)

# run-lengthの左右を見て合わせる
res = 0
i = 0
while i < len(rl) // 2:
    res += max(rl[i], rl[len(rl) - i - 1]) - min(rl[i], rl[len(rl) - i - 1])
    i += 1
print(res)