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)

#![allow(unused_imports)]

use std::cmp::{self, Ordering};
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};
use std::fmt::{self, Debug, Display};
use std::io::{self, Read, Write};
use std::process;
use std::str::{self, FromStr};


fn solve(s: String) -> Option<usize> {
    let s = s.as_bytes();
    let (mut i, mut j, mut r) = (0, s.len() - 1, 0);
    loop {
        if i >= j {
            return Some(r);
        }
        if s[i] == s[j] {
            i += 1;
            j -= 1;
        } else if s[i] == 'x' as u8 {
            i += 1;
            r += 1;
        } else if s[j] == 'x' as u8 {
            j -= 1;
            r += 1;
        } else {
            return None;
        }
    }
}


fn main() {
    let stdin = io::stdin();
    let mut sc = Scanner::new(stdin.lock().bytes());
    match solve(sc.next()) {
        Some(n) => println!("{}", n),
        None => println!("-1"),
    }
}


struct Scanner<R: Read> {
    buf: Vec<u8>,
    bytes: io::Bytes<R>,
}

#[allow(dead_code)]
impl<R: Read> Scanner<R> {
    fn new(bytes: io::Bytes<R>) -> Self {
        Scanner {
            buf: Vec::with_capacity(256),
            bytes: bytes,
        }
    }

    fn next<T: FromStr>(&mut self) -> T
    where
        <T as FromStr>::Err: Debug,
    {
        self.buf.clear();
        for c in self.bytes
            .by_ref()
            .map(|c| c.unwrap())
            .skip_while(|&c| c == ' ' as u8 || c == '\n' as u8)
            .take_while(|&c| !(c == ' ' as u8 || c == '\n' as u8))
        {
            self.buf.push(c);
        }
        str::from_utf8(&self.buf).unwrap().parse().unwrap()
    }

    fn vec<T: FromStr>(&mut self, n: usize) -> Vec<T>
    where
        <T as FromStr>::Err: Debug,
    {
        (0..n).map(|_| self.next()).collect()
    }

    fn pairs<A: FromStr, B: FromStr>(&mut self, n: usize) -> Vec<(A, B)>
    where
        <A as FromStr>::Err: Debug,
        <B as FromStr>::Err: Debug,
    {
        (0..n).map(|_| (self.next(), self.next())).collect()
    }

    fn trios<A: FromStr, B: FromStr, C: FromStr>(&mut self, n: usize) -> Vec<(A, B, C)>
    where
        <A as FromStr>::Err: Debug,
        <B as FromStr>::Err: Debug,
        <C as FromStr>::Err: Debug,
    {
        (0..n)
            .map(|_| (self.next(), self.next(), self.next()))
            .collect()
    }

    fn mat<T: FromStr>(&mut self, m: usize, n: usize) -> Vec<Vec<T>>
    where
        <T as FromStr>::Err: Debug,
    {
        (0..n).map(|_| self.vec(m)).collect()
    }
}


#[allow(dead_code)]
fn print_yesno(x: bool, yes: &'static str, no: &'static str) {
    println!("{}", if x { yes } else { no });
}


#[allow(dead_code)]
fn print_foreach<T: Display, I: IntoIterator<Item = T>>(xs: I) {
    for x in xs.into_iter() {
        println!("{}", x);
    }
}


#[allow(dead_code)]
fn print_or<T: Display>(x: Option<T>, or: &'static str) {
    if let Some(x) = x {
        println!("{}", x);
    } else {
        println!("{}", or);
    }
}