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
n = s.length
l = 0
r = n - 1
ans = 0
cnt = 0
while true
  break if cnt >= n
  if s[l] == "x" && s[r] == "x"
    l += 1
    r -= 1
    cnt += 1
  elsif s[l] == "x" && s[r] != "x"
    ans += 1
    l += 1
    cnt += 1
  elsif s[l] != "x" && s[r] == "x"
    ans += 1
    r -= 1
    cnt += 1
  else
    if s[l] == s[r]
      l += 1
      r -= 1
      cnt += 2
    else
      puts -1
      exit
    end
  end
end
puts ans