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 (3.8.2) (judge verdict: Time Limit Exceeded)

import collections

s = input()
slen = len(s)
swox = s.replace('x','')
swox_rev = swox[::-1]
if swox != swox_rev:
    print(-1)
else:
    sl = list(s)
    sl_inv = list(s[::-1])
    cnt = 0
    while sl:
        if sl[0]== sl_inv[0]:
            sl.pop(0)
            sl_inv.pop(0)
        elif sl[0] == 'x' and sl_inv[0] !='x':
            sl.pop(0)
        elif sl[0] != 'x' and sl_inv[0] == 'x':
            cnt += 1
            sl_inv.pop(0)
        if sl_inv ==[]:
            break
        #print(sl,sl_inv,cnt)
    print(cnt+len(sl_inv))