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: Accepted)

#include <cstring>
#include <cstdint>
#include <iostream>
#include <string>
#include <memory>
#include <queue>
#include <limits>
#include <list>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <utility>
#include <sstream>
#include <random>
#include <functional>
#include<algorithm>
using namespace std;

int main() {
	string str;
	cin >> str;
	size_t l = 0,r = str.size() - 1;
	int v = 0;
	while(l < r)
	{
		while ((str[l] == str[r]) && (l < r))
		{
			l++;r--;
		}
		if (l < r) {
			if (str[l] == 'x') {
				v++;
				l++;
			}
			else if (str[r] == 'x') {
				v++;
				r--;
			}
			else {
				v = -1;
				break;
			}
		}
	}
	cout << v;
	return 0;
}