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)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

#define LINEMAX 100002

int main(int argc, char *argv[]){

	char line[LINEMAX];

	fgets(line, sizeof(line), stdin);

	char *fromHead = line;
	char *fromTail = &line[strlen(line)-2];
	int   sunukeCnt = 0;

	//printf("fromHead is %c\n", *fromHead);
	//printf("fromTail is %c\n", *fromTail);

	while(1){

		if(fromTail == fromHead || (fromTail+1) == fromHead){
			break;
		}

		if(*fromHead == *fromTail){
			fromHead++;
			fromTail--;
			continue;
		}

		if(*fromHead == 'x'){
			sunukeCnt++;
			fromHead++;
			continue;
		}

		if(*fromTail == 'x'){
			sunukeCnt++;
			fromTail--;
			continue;
		}

		sunukeCnt = -1;
		break;
	}

	printf("%d\n", sunukeCnt);
	return 0;
}