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: Accepted)

def main():
    import sys

    def input(): return sys.stdin.readline().rstrip()

    s = input()
    n = len(s)
    sr = s.replace('x', '')
    if sr == '':
        print(0)
        return
    elif sr != sr[::-1]:
        print(-1)
        return
    cur_l = 0
    cur_r = n-1
    ans = 0
    cnt = 0
    while cur_l < cur_r:
        while s[cur_l] == 'x':
            cnt += 1
            cur_l += 1
        while s[cur_r] != s[cur_l]:
            cur_r-=1
            cnt -= 1
        ans += abs(cnt)
        cur_l += 1
        cur_r -= 1
        cnt = 0
        
    print(ans)   
    
        

if __name__ == '__main__':
    main()