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Check Digit Verification of CAS Registry Numbers

A CAS Registry Number includes up to 10 digits which are separated into 3 groups by hyphens. The first part of the number, starting from the left, has up to 7 digits; the second part has 2 digits. The final part consists of a single check digit.

The check digit is developed by following a standard calculation shown below. Use it to see if you have a valid CAS Registry Number.

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	The CAS Registry Number may be written in a general form as:


	 N ......N N - N N - R
	  i       4 3   2 1

in which R represents the check digit and N  represents a fundamental
sequential number. The check digit is derived from the following formula


	iN  + ... + 4N + 3N + 2N + 1N          R
	  i           4    3    2    1
	-------------------------------- = Q + --

			10                     10


in which Q represents an integer which is discarded.


	For example, RN 107-07-3, the validity is checked as follows:


	(5 x 1) + (4 x 0) + (3 x 7) + (2 x 0) + (1 x 7)    33          3
       ------------------------------------------------- = -- = 3  +  ----
				10                         10          10


Q = 3 is discarded and R (the check digit) = 3