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: Wrong Answer)

s = gets.chomp
rev_s = s.reverse
delete_x = String.new(s.delete("x"))
cnt = 0
flag = true
if delete_x != delete_x.reverse
  flag = false
end
i = j = 0
while (i < (s.size - 1) / 2) || (j < (s.size - 1) / 2)
  if s[i] == "x" && rev_s[j] != "x"
    cnt += 1
    i += 1
  elsif s[i] != "x" && rev_s[j] == "x"
    cnt += 1
    j += 1
  else
    i += 1
    j += 1
  end

end
puts flag ? cnt : -1