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 Go (1.6) (judge verdict: Accepted)

///
// File:  c.go
// Author: ymiyamoto
//
// Created on Fri Jun  1 21:57:47 2018
//
package main

import (
	"fmt"
	"strings"
)

var s string

func isPalidrome(s string) bool {
	for i := range s {
		if s[i] != s[len(s)-1-i] {
			return false
		}
	}
	return true
}

func main() {
	fmt.Scan(&s)

	t := strings.Replace(s, "x", "", -1)
	if !isPalidrome(t) {
		fmt.Println(-1)
		return
	}
	ans := 0
	left := 0
	right := len(s) - 1
	for left < len(s) || 0 <= right {
		if left < len(s) && 0 <= right && s[left] == s[right] {
			left++
			right--
		} else if left < len(s) && s[left] == 'x' {
			ans++
			left++
		} else if 0 <= right && s[right] == 'x' {
			ans++
			right--
		}
	}

	fmt.Println(ans / 2)
}