Summarize the algorithm described between START and STOP
START
This is the process:
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Take the first 12 digits of the 13-digit ISBN
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Multiply each number in turn, from left to right by a number. The first digit is multiplied by 1, the second by 3, the third by 1 again, the fourth by 3 again, and so on to the eleventh which is multiplied by 1 and the twelfth by 3.
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Add all of the 12 answers.