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: Accepted)

ans = 0
s = input()

s_without_x = [ i for i in s if i != "x" ]
if s_without_x != s_without_x[::-1]:
    ans = -1

else:
    x_list = []
    x = 0
    for i, v in enumerate(s):
        if v != "x":
            x_list.append(x)
            x = 0
        else:
            x += 1
        if i == len(s) - 1:
            x_list.append(x)

    for i in range(len(x_list)//2):
        a, b = max(x_list[i], x_list[-1-i]), min(x_list[i], x_list[-1-i])
        ans += a-b

print(ans)