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;
use proconio::marker::Chars;

fn main() {
    input! {
        s: Chars,
    };
    if (0..s.len() / 2).all(|i| s[i] == s[s.len() - i - 1]) {
        println!("0");
        return;
    }
    let nx = s
        .iter()
        .filter(|&&c| c != 'x')
        .map(|&c| c)
        .collect::<Vec<char>>();
    if !(0..nx.len() / 2).all(|i| nx[i] == nx[nx.len() - i - 1]) {
        println!("-1");
        return;
    }
    let mut ans = 0;
    let mut i_l = 0;
    let mut i_r = s.len() - 1;
    while i_l < i_r {
        let s_l = s[i_l];
        let s_r = s[i_r];
        if s_l == s_r {
            i_l += 1;
            i_r -= 1;
        } else if s_l == 'x' {
            i_l += 1;
            ans += 1;
        } else if s_r == 'x' {
            i_r -= 1;
            ans += 1;
        } else {
            unreachable!();
        }
    }
    println!("{}", ans);
}