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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
using namespace std;
string a;
string b;
int main()
{
    cin>>a;
    for(auto it:a)
    {
        if(it!='x')b+=it;
    }
    bool ok=1;
    for(int st=0,dr=b.size()-1;st<dr;st++,dr--)
        ok&=(b[st]==b[dr]);
    if(!ok){cout<<-1;return 0;}
    int rez=0;
    for(int st=0,dr=a.size()-1;st<dr;)
    {
        if(a[st]==a[dr]){st++;dr--;}
        else
        {
            rez++;
            if(a[st]=='x')st++;
            else dr--;
        }
    }
    cout<<rez;
    return 0;
}