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)

s=gets.chomp
t=s.delete("x")
def r?(s)
  t=s.dup
  while true
    case t.size
    when 0,1
      return true
    else
      unless t.slice!(0).eql?(t.slice!(-1))
        return false
      end
    end
  end
end
unless r?(t)
  puts -1
  exit
end

i=0
f=0
l=s.size-1
while true
  if f==l
    puts i
    exit
  end
  if s[f]==s[l]
    if f+1==l
      break
    else
      f+=1
      l-=1
      next
    end
  elsif s[f].eql?("x")
    (f+=1;i+=1) while s[f].eql?("x")
    break if f+1==l
  elsif s[l].eql?("x")
    (l-=1;i+=1) while s[l].eql?("x")
    break if f+1==l
  end
end
puts i