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: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace C
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            string x_elim = new string(input.Where(x => x != 'x').ToArray());
            int ans = -1;
            if (x_elim.Substring(0, x_elim.Length / 2) == x_elim.Substring(x_elim.Length / 2 + x_elim.Length % 2, x_elim.Length / 2))
            {
                ans++;
                //is Palindrome
                int i = 0;
                int j = input.Length - 1;
                while (true)
                {
                    while (input[i] == 'x')
                    {
                        if (i == j)
                            goto end;
                        if (input[j] == 'x')
                        {
                            i++;
                            j--;
                            if (i >= j)
                                goto end;
                        }
                        else
                        {
                            i++;
                            ans++;
                            if (i >= j)
                                goto end;
                        }
                    }

                    while (input[j] == 'x')
                    {
                        if (i == j)
                            goto end;
                        if (input[i] == 'x')
                        {
                            i++;
                            j--;
                            if (i >= j)
                                goto end;
                        }
                        else
                        {
                            j--;
                            ans++;
                            if (i >= j)
                                goto end;
                        }
                    }

                    i++;
                    j--;
                    if (i >= j)
                        goto end;
                }
            }

            end:
            Console.WriteLine(ans);

#if DEBUG
            Console.ReadKey();
#endif
        }
    }
}