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: Accepted)

#  --*-coding:utf-8-*--

def f(str):
    numOfX = 0
    numOfXs = []
    letters = []
    
    for c in str:
        if c == 'x':
            numOfX += 1
        else:
            letters.append(c)
            numOfXs.append(numOfX)
            numOfX = 0
            
    numOfXs.append(numOfX)

    for i in range(len(letters)//2):
        if letters[i] != letters[-i-1]:
            return -1

    a = 0
    for i in range(len(numOfXs)//2):
        a += abs(numOfXs[i] - numOfXs[-i-1])
            
    return a

def test():
    assert f('xabxa') == 2
    assert f('ab') == -1
    assert f('a') == 0
    assert f('oxxx') == 3


def main():
    str = input()
    print(f(str))

if __name__ == '__main__':
    main()