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: Wrong Answer)

package main

import "fmt"

func main() {
	var s string
	fmt.Scan(&s)
	ans := 0
	j := len(s) - 1
	ok := true
	for i := 0; i < len(s); {
		if s[i] != s[j] {
			if s[i] == 'x' {
				// 後ろに'x'があったことにしてiだけ進める
				ans++
				i++
			} else if s[j] == 'x' {
				// 前に'x'があったことにしてjだけ進める
				if j > 0 {
					j--
				}
				ans++
			} else {
				ok = false
				break
			}
		} else {
			i++
			if j > 0 {
				j--
			}
		}
		if i == j {
			break
		}
	}
	if ok {
		fmt.Println(ans)
	} else {
		fmt.Println(-1)
	}
}