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: Accepted)

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		String s2 = s.replace("x", "");
		if (!s2.equals(new StringBuilder(s2).reverse().toString())) {
			System.out.println(-1);
			return;
		}
		char[] t = s.toCharArray();
		int n = t.length;
		int p = 0;
		int q = n - 1;
		int ans = 0;
		while(p < q) {
			int left = 0;
			int right = 0;
			while(p < n && t[p] == 'x') {
				p++;
				left++;
			}
			while(q >= 0 && t[q] == 'x') {
				q--;
				right++;
			}
			if (p <= q) {
//				if (t[p] != t[q]) {
//					throw new RuntimeException();
//				}
				ans += Math.abs(left - right);
			}
			p++;
			q--;
		}
		System.out.println(ans);
	}
}