KURSCHAK 1974 Problem 1
A library has one exit and one entrance and a blackboard at each. Only one person enters or leaves at a time. As he does so he records the number of people found/remaining in the library on the blackboard. Prove that at the end of the day exactly the same numbers will be found on the two blackboards (possibly in a different order).
Dataset text AI-MO AoPS. Quoted with source.
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