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>
#include <map>
#include <string>

using namespace std;

struct node{
    int num, pos;
};

int main(){
    string S; cin >> S;

    int l = 0, r = S.size()-1;
    int cnt = 0;

    while(l < r){
        if(S[l] == S[r]){
            l++; r--;
        }else{
            if(S[l] != 'x' && S[r] != 'x'){
                cout << -1 << endl;
                return 0;
            }
            if(S[l] == 'x'){
                cnt++;
                l++;
            }else if(S[r] == 'x'){
                cnt++;
                r--;
            }
        }
    }

    cout << cnt << endl;

    return 0;

}