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Consider an urn containing 1996 slips of papers numbered from 1 to 1996. A procedure consists of three steps:
1. Take two slips from the urn.
2. Write down the absolute value of the difference of the numbers on the slips of step1 on a new slip of paper.
3. Discard the two slips in step1 and replace them with the slip in step2 into the urn.
Repeating the procedure until only one slip of paper is left in the urn, prove that the number on this slip is even.