pub fn max_value(mut n: isize, digit: char) -> Result<isize, String> {
    let digit = match digit.to_digit(10) {
        Some(d) => Ok(d as isize),
        None => Err("The digit is not valid"),
    }?;

    let negative = n < 0;
    if negative {
        n *= -1;
    }

    let nlen = (n.abs() as f32).log10().floor() as u32 + 1;
    let mut max = n * 10 + digit;

    let cmp = if negative {
        std::cmp::min
    } else {
        std::cmp::max
    };

    for i in 1..=nlen {
        let option = (n / 10isize.pow(i) * 10 + digit) * 10isize.pow(i) + n % 10isize.pow(i);
        max = cmp(max, option);
    }

    Ok(if negative { -max } else { max })
}