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 C (GCC 5.4.1) (judge verdict: Accepted)

// 再提出

#include <stdio.h>
 
char s[100001];
 
int main(void) {
	int len;
 
	scanf("%s%n", s, &len);
 
	int ans = 0;
	for (int l = 0, r = len - 1; l <= r; ) {
		if (s[l] == s[r]) {
			l++;
			r--;
		} else if (s[l] == 'x') {
			l++;
			ans++;
		} else if (s[r] == 'x') {
			r--;
			ans++;
		} else {
			puts("-1");
			return 0;
		}
	}
 
	printf("%d\n", ans);
 
	return 0;
}