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)

#!/usr/bin/env ruby
s = gets.strip

unless s.chars.reject { |c| c == 'x' }.tap { |k| break (k == k.reverse) }
  puts '-1'
  exit
end

result = 0
ue = s.chars
st = ue.reverse

while !ue.empty? || !st.empty?
  if ue.first == st.first
    ue = ue.drop(1)
    st = st.drop(1)
  else
    if ue.first == 'x'
      result += 1
      ue = ue.drop(1)
    else
      result += 1
      st = st.drop(1)
    end
  end
end

puts result / 2