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

rem = s.delete("x")
if rem != rem.reverse
  puts "-1"
  exit
end

bit = rem.size / 2
first = []
last = []
index = 0
s.split("").each do |c|
  if c != "x"
    index +=1
  end
  if index <= bit
    first << c
  else
    last << c
  end
end

#p first
last.reverse!

if rem.size % 2 != 0
  last.slice!(-1)
end

#p last

i = 0
im = 0
count = 0
while true
  if first[i] != last[i]
    if first[i] != "x"
      first.insert(i,"x")
    elsif last[i] != "x"
      last.insert(i,"x")
    end
    count += 1
  end
  i+=1
#  puts "----------"
#  p first
#  p last
  if first == last
    break
  end
#  gets
end
puts count
exit