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 (1.15.1) (judge verdict: Accepted)

pub fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

pub fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}

pub fn read_vec2<T: std::str::FromStr>(n: usize) -> Vec<Vec<T>> {
    (0..n).map(|_| read_vec()).collect()
}

pub fn read_col<T: std::str::FromStr>(n: usize) -> Vec<T> {
    (0..n).map(|_| read()).collect()
}

fn diff(a: u32, b: u32) -> u32 {
    if a > b {
        a - b
    } else {
        b - a
    }
}

fn main() {
    let s: String = read();
    let sv: Vec<char> = s.chars().collect();

    let mut cv: Vec<char> = vec![];
    let mut xv: Vec<u32> = vec![];
    let mut i: usize = 0;
    let mut cnt = 0;
    while i < sv.len() && sv[i] == 'x' {
        i += 1;
        cnt += 1;
    }
    while i < sv.len() {
        if sv[i] == 'x' {
            cnt += 1;
        } else {
            xv.push(cnt);
            cnt = 0;
            cv.push(sv[i]);
        }
        i += 1;
    }
    xv.push(cnt);

    for i in 0..cv.len() {
        if i >= cv.len() - 1 - i {
            break;
        }
        if cv[i] != cv[cv.len() - 1 - i] {
            println!("-1");
            return;
        }
    }

    let mut ans: u32 = 0;
    for i in 0..xv.len() {
        if i >= xv.len() - 1 - i {
            break;
        }
        ans += diff(xv[i], xv[xv.len() - 1 - i]);
    }

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