The International Standard Book Number (ISBN) is a numeric commercial book identifier that is intended to be unique.

It used to be to 10-digit long. Recently it is changed to 13-digit.

For example, a= '0-306-40615-2' is in ISBN-10 format.

ISBN-10 check digit (which is the last digit of the 10-digit ISBN) must range from 0 to 10 (the symbol 'X' is used for 10) and must be such that the sum of the ten digits, each multiplied by its (integer) weight, descending from 10 to 1, is a multiple of 11.

ISBN-13 uses a different approach to calculate the check digit.

For this problem, u've to calculate the check digit for ISBN-10.