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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <math.h>
#define Max(a, b) ((a) > (b) ? (a) : (b))
#define Min(a, b) ((a) > (b) ? (b) : (a))
#define Abs(x) ((x) > 0 ? (x) : -(x))
#define REP(i, n) for(int i = 0; i < n; i++)
#define INF 1000000000000 //10^12
#define MOD 1000000007 //10^9 + 7
typedef long long ll;

int
main(int arg, char *argv[])
{
  char s[200000];
  scanf("%s", s);
  int f = 0, l = strlen(s) - 1, cnt = 0, flag = 0;
  while(l - f >= 0){
    if(s[f] == 'x' && s[l] != 'x'){
      f++;
      cnt++;
    }
    else if(s[f] != 'x' && s[l] == 'x'){
      l--;
      cnt++;
    }
    else if(s[l] == s[f]){
      f++;
      l--;
    }
    else{
      flag++;
      break;
    }
  }
  if(flag == 0) printf("%d\n", cnt);
  else printf("-1\n");

  return 0;
}