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 Rust (1.42.0) (judge verdict: Accepted)

use proconio::{input, marker::Chars};
use std::collections::VecDeque;

fn main() {
    input! {
        s: Chars,
    }

    let mut deque = VecDeque::from(s);
    let mut ans = 0;

    while let (Some(front), Some(back)) = (deque.pop_front(), deque.pop_back()) {
        if front == back {
            continue;
        }
        if front == 'x' {
            deque.push_back(back);
            ans += 1;
        } else if back == 'x' {
            deque.push_front(front);
            ans += 1;
        } else {
            println!("-1");
            return;
        }
    }

    println!("{}", ans);
}