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: Wrong Answer)

#include<iostream>
#include<string>
using namespace std;
int main() {
	string s;
	cin >> s;
	int start = 0;
	int goal = s.size()-1;
	int ans = 0;
	while (true) {
		if (start > goal)break;
		if (s == s) {
			break;
		}
		if (s[start]!=s[goal]) {
			if (s[start] == 'x') {
				ans++;
				start++;
			}
			else if(s[goal]=='x') {
				ans++;
				goal--;
			}
			else {
				cout << -1 << endl;
				return 0;
			}
		}
		if (start > goal)break;
		start++;
		goal--;
	}
	cout << ans << endl;
	return 0;
}