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def is_valid(isbn):
    isbn = isbn.replace("-", "")

    if len(isbn) != 10 or (isbn[-1] != "X" and not isbn[-1].isnumeric()):
        return False

    check = isbn[-1]

    if check == "X":
        check = 10
    else:
        check = int(check)

    sum = 0

    for i, digit in enumerate(isbn[:-1]):
        if not digit.isnumeric():
            return False

        sum += int(digit) * (10 - i)


    return True if ((sum + check) % 11) == 0 else False