def is_valid(isbn):
    # remove dashes
    isbn_no_dashes: str = "".join(isbn.split("-"))
    sum: int = 0
    # is isbn empty
    if isbn == "":
        return False
    # isbn length must be 10
    if len(isbn_no_dashes) < 10 or len(isbn_no_dashes) > 10:
        return False
    for index, digit in enumerate(isbn_no_dashes):
        # non-digit in isbn not as a checksum, return False
        if digit.isalpha() and index != 9:
            return False
        # if its a digit, ok
        if digit.isdigit():
            sum += int(digit) * (10 - index)
        # if check digit is not digit or not an 'X', return False
        if index == 9 and digit.isalpha() and digit.lower() != "x":
            return False
        # if check digit is the letter 'X' treat it as a 10
        if digit.isalpha() and index == 9 and digit.lower() == "x":
            sum += 10 * (10 - index)
    return False if sum % 11 else True