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: Runtime Error)

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

namespace C_Insertingx
{
    class Program
    {
        static void Main(string[] args)
        {

            string strS = Console.ReadLine();

            if (strS.Length == 1) {
                Console.WriteLine(0);
                return;
            }

            string strRep = strS.Replace("x", "");

            int intHalfL = strRep.Length / 2;

            string strComp1 = strRep.Substring(0, intHalfL);

            // 編集文字列を末尾からつなげて回文作成
            string strComp2 = "";
            int intLast = strRep.Length;
            for (int i = 0; i < intHalfL; i++ ) {
                strComp2 += strRep.Substring(intLast, 1);
                intLast--;
            }

            if (strComp1 != strComp2) {
                Console.WriteLine(-1);
                return;
            }

            int intHalfL2 = strS.Length / 2;

            string strShalf = strS.Substring(0, intHalfL2);

            int intCnt = 0;
            for (int i = 0; i < strShalf.Length; i++) {
                if (strShalf.Substring(i,1) == "x") {
                    intCnt++;
                }
            }

            int intCnt2 = 0;
            int intLast2 = strShalf.Length;
            for (int i = 0; i < strShalf.Length; i++)
            {
                if (strShalf.Substring(intLast2, 1) == "x")
                {
                    intCnt2++;
                }
                intLast2--;
            }

            Console.WriteLine(Math.Abs(intCnt - intCnt2));

        }
    }
}