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)

def rev?(s) return s.start_with?(s[(s.size/2)..-1].reverse) end

s = gets.chomp
sx = s.gsub('x', '')
res = rev? sx
if res
  x_num = []
  count = 0
  s.each_char{|c|
    if c == 'x'
      count += 1
    else
      x_num << count
      count = 0
    end
  }
  if sx.size == 0
    p 0
  else
    if s[-1] != 'x'
      x_num << 0 
    else
      x_num << count
    end
    lx = x_num[0...(x_num.size/2)]
    rx = x_num[-(x_num.size/2)..-1].reverse
    if lx.size == 1
      p (lx[0] - rx[0]).abs
    else
      p [lx, rx].map{|l, r| (l - r).abs}.inject :+
    end
  end
else
  p -1
end