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)

/**
 *    author:  tourist
 *    created: 22.10.2017 08:48:47       
**/
#include <bits/stdc++.h>

using namespace std;

int main() {
  string s;
  cin >> s;
  int n = s.length();
  string w = "";
  for (int i = 0; i < (int) s.length(); i++) {
    if (s[i] != 'x') {
      w += s[i];
    }
  }
  string ww = w;
  reverse(ww.begin(), ww.end());
  if (w != ww) {
    printf("%d\n", -1);
    return 0;
  }
  int ans = 0;
  int i = 0, j = n - 1;
  while (i < j) {
    if (s[i] == s[j]) {
      i++;
      j--;
      continue;
    }
    ans++;
    if (s[i] == 'x') {
      i++;
    } else {
      j--;
    }
  }
  printf("%d\n", ans);
  return 0;
}