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# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

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

namespace ConsoleApp15
{
    class Class407
    {
        static void Main()
        {
            string S = Console.ReadLine();
            string x1 = string.Join("", S.Where(_ => _ != 'x'));
            string x2 = string.Join("", x1.Reverse());
            if (x1 != x2) { Console.WriteLine(-1);return; }
            List<char> s = S.ToList();

            int l = 0;
            int count = 0;
            for (int i = 0; i < S.Length; i++)
            {
                if (s[i] == 'x' && s[S.Length - 1 - i] != 'x')
                {
                    s.Insert(S.Length - i, 'x');
                    count++;
                }
                else if(s[S.Length-1-i+l] == 'x' && s[i] != 'x')
                {
                    s.Insert(i+l, 'x');
                    count++;
                    l++;
                }
            }
            Console.WriteLine(count);

        }
    }

}