ISBN_LENGTH = 10


def is_valid(isbn):

    isbn = isbn.replace('-', '')
    if len(isbn) != ISBN_LENGTH:
        return False

    checksum = 0

    for ch, i in zip(isbn, range(ISBN_LENGTH, 1, -1)):
        if not ch.isdigit():
            return False

        checksum += int(ch)*i

    ch = isbn[-1]

    if ch.isdigit():
        x = int(ch)
    elif ch == 'X':
        x = 10
    else:
        return False

    checksum += x

    return checksum % 11 == 0