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++14 (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

#include <bits/stdc++.h>

using namespace std;

int main()
{
	char s[100006];
	scanf("%s",s);
	int a[26] = {0};
	int n = strlen(s);
	for(int i = 0;i<n;i++)
		a[s[i]-'a'] += 1;
	int flag = 0;
	for(int i = 0;i<26;i++)
	{
		if(i == 'x'-'a')
			continue;
		flag += (a[i]%2);
	}
	if(flag > 1)
	{
		cout << -1 << endl;
		return 0;
	}

	int left = 0;
	int right = n-1;
	int ans = 0;
	while(left < right)
	{
		if(s[left] == s[right])
		{
			left++;
			right--;
		}
		else
		{
			if(s[left] == 'x')
				ans++,left++;
			else if(s[right] == 'x')
				ans++,right--;
			else
				flag = -1;

		}
	}

	if(flag == -1)
		cout << flag << endl;
	else
		cout << ans << endl;
	return 0;
}