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 <bits/stdc++.h>
using namespace std;
typedef long long LL;

int main(){
    string str;
    cin >> str;
    int s = 0;
    int e = str.size()-1;
    int ok = 1;

    int ans = 0;

    while(s <= e){
        if(s == e){
            break;
        }
        if(str[s] == str[e]){
            s++;
            e--;
            continue;
        }
        if(str[s] == 'x'){
            s++;
            ans++;
            continue;
        }
        if(str[e] == 'x'){
            e--;
            ans++;
            continue;
        }
        ok = 0;
        break;
    }
    if(ok == 0){
        cout << -1 << endl;
    } else {
        cout << ans << endl;
    }
}