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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;

namespace code_festival_2017_qualc_c
{
	class Program
	{
		static void Main(string[] args)
		{
			string s = Console.ReadLine();
			string[] arr = s.Split('x');
			string a = "";
			for (int i = 0; i < arr.Length; i++) a += arr[i];
			char[] c = a.ToCharArray(); Array.Reverse(c);
			if (a == new string(c))
			{
				List<int> list = new List<int>();
				int count = 0;
				for (int i = 0; i < s.Length; i++)
				{
					if (s[i] == 'x') count++;
					else
					{
						list.Add(count);
						count = 0;
					}
				}
				list.Add(count);
				int ret = 0;
				for (int i = 0; i < list.Count; i++)
				{
					ret += Math.Abs(list[i] - list[list.Count - i - 1]);
				}
				Console.WriteLine(ret / 2);
			}
			else
			{
				Console.WriteLine(-1);
			}
		}
	}
}