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: Wrong Answer)

import java.util.Scanner;

public class Main {
    static Scanner in;

    void solve() {
        String s = in.nextLine();
        StringBuilder sb = new StringBuilder();
        sb.append(s.replace("x", ""));
        String s2 = sb.toString();
        if (s.equals(s2)) {
            System.out.println(0);
            return;
        }
        if (!s.replace("x", "").equals(s2)) {
            System.out.println(-1);
            return;
        }
        int left = 0, right = s.length() - 1, ans = 0;
        while(left < right) {
            if(s.charAt(left) == s.charAt(right)) {
                left++;
                right--;
            }else{
                ans++;
                if(s.charAt(left) == 'x') {
                    left++;
                }else {
                    right--;
                }
            }
        }
        System.out.println(ans);
    }

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