Sixteen counters, which are black on one side and white on the other, are arranged in a by square. Initially all the counters are facing black side up. In one 'move', you must choose a by square within the square and turn all four counters over once. Describe a sequence of 'moves' of minimum length that finishes with the colours of the counters of the by square alternating (as shown in the diagram).
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Junior Mathematical Olympiad 2018 (2018)
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