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.Threading.Tasks;

namespace AtCoder_Project_CSharp
{
    class Program
    {
        static void Main(string[] args)
        {
            var s = Console.ReadLine();
            if (s.Length == 1)
            {
                Console.WriteLine(0);
                return;
            }

            var s_rev = new string(s.Reverse().ToArray());//.ToArray();
            if(s==s_rev)
            {
                Console.WriteLine(0);
                return;
            }


            for (int i = 0; i < s.Length; i++)
            {
                char center = s[i];
                if (i + 1 >= s.Length) break;
                string front_half = s.Substring(0, i);
                string back_half = s.Substring(i + 1);
                string back_half_rev = new string(back_half.Reverse().ToArray());
                if(front_half.Replace("x","")==back_half_rev.Replace("x",""))
                {
                    int count = front_half.Count(e => e == 'x') - back_half_rev.Count(e => e == 'x');
                    if(count==0)
                    {
                        Console.WriteLine(front_half.Count(e => e == 'x') * 2);
                        return;
                    }
                    Console.WriteLine(Math.Abs(count));
                    return;
                }
            }
            for (int i = 0; i < s.Length; i++)
            {
                if (i + 1 >= s.Length) break;
                string front_half = s.Substring(0, i);
                string back_half = s.Substring(i);
                string back_half_rev = new string(back_half.Reverse().ToArray());
                if (front_half.Replace("x", "") == back_half_rev.Replace("x", ""))
                {
                    int count = front_half.Count(e => e == 'x') - back_half_rev.Count(e => e == 'x');
                    if (count == 0)
                    {
                        Console.WriteLine(front_half.Count(e => e == 'x') * 2);
                        return;
                    }
                    Console.WriteLine(Math.Abs(count));
                    return;
                }
            }
            Console.WriteLine(-1);
        }
    }
}