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)

def is_palindrome str
  str == str.reverse
end
 
def solve s, t
  
  return  0 if is_palindrome(s) 
  return -1 unless is_palindrome(t)
  
  tl = t.length
  o = (tl+1)/2 - 1
  i = j = k = l = m = cnt = 0
  
  r = s.reverse
  
  loop do
      
    while s[i] == 'x' do
      i+=1; k+=1;
    end
    
    while r[j] == 'x' do
      j+=1; l+=1;
    end
    
    cnt += ( k - l ).abs
    
    k = l = 0

    break if m == o
    
    i += 1; j += 1; m += 1
  end
  
  return cnt
end
 
s = gets.chomp
t = s.gsub("x","")
 
puts solve s, t