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 Ruby (2.3.3) (judge verdict: Accepted)

input = $stdin.read.split("\n")

input.each do |line|
    a = line.split("")
    nx = a.select {|c| c!="x"}

    f = nx[0...nx.length/2]
    l = nx[(nx.length-nx.length/2)...(nx.length)].reverse

    if f==l
        n = 0
        i = 0
        j = a.length-1
        while i <= j
            # p [a[i], a[j]]
            if a[i] == a[j]
                i += 1
                j -= 1
            elsif a[i] == 'x'
                i += 1
                n += 1
            else
                j -= 1
                n += 1
            end
        end
        puts n
    else
        puts -1
    end
end