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 Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.util.ArrayList;
import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        String s = scanner.next();

        if (isPalindrome(s)) {
            System.out.println(0);
            return;
        }

        int xCount = 0;
        String newStr = "";
        ArrayList<Integer> xPos = new ArrayList<>();

        for (int i = 0; i < s.length(); i++) {
            String tmp = s.substring(i, i + 1);
            if (tmp.equals("x")) {
                xCount++;
            } else {
                newStr += tmp;
                xPos.add(xCount);
                xCount = 0;
            }
        }
        xPos.add(xCount);

        int ans = 0;

        for (int i = 0; i < xPos.size() / 2; i++) {
            ans += Math.abs(xPos.get(i) - xPos.get(xPos.size() - 1 - i));
        }

        if (isPalindrome(newStr)) {
            System.out.println(ans);
        } else {
            System.out.println(-1);
        }
    }


    private static boolean isPalindrome(String sentence) {
        StringBuilder sb = new StringBuilder(sentence);
        return sentence.equals(sb.reverse().toString());
    }
}