ISBN (International Standard Book Number)
is a unique number assigned
to each book.
The number has 9 information digits and ends with 1 check
digit.
Assuming the digits are "abcdefghi-j" where j is the check digit. Then
the check digit is computed by the following formula:
j = [a b c d e f g h i] * [1 2 3 4 5 6 7 8 9]' (mod 11)