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)

def check(s)
    i=0
    s.delete("x")
    ((s.length)/2).floor.times{
        if !(s[i]==s[-i-1])
            p (s[i]+" "+s[-i-1])
            print("-1")
            exit
        else
            i+=1
        end
    }
end

def make(s)
    i=0
    j=0
    while i<((s.length)/2).floor do
        if !(s[i]==s[-i-1])
            if s[-i-1]=="x"
                s.insert(i,"x") 
            else
            s.insert(-i-1,"x")
            end
            j+=1
        else
            i+=1
        end
    end
    return j
end


s=gets.chomp.split("")
t=s.clone
check(t)
print(make(s))