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 Scala (2.11.7) (judge verdict: Accepted)

object Main extends App {
  val s = io.StdIn.readLine()
  val check = s.filterNot(_=='x')

  if(check!=check.reverse){
    println(-1)
    sys.exit()
  }

  var ans = 0
  var first = 0
  var latter = 0
  var i = 0
  while( i < (s.length + first + latter)/2){
    if( s(i - first) != s(-1+(-1)*i+latter+s.length) ){
      ans += 1
      if( s(i-first)=='x' ){
        latter += 1
      }else{
        first += 1
      }
    }
    i += 1
  }

  println(ans)

}