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BG-MO 2022 Problem 1

    A white equilateral triangle TT with side length 20222022 is divided into equilateral triangles with side 11 (cells) by lines parallel to the sides of TT . We'll call two cells adjacent\textit{adjacent} if they have a common vertex. Ivan colours some of the cells in black. Without knowing which cells are black, Peter chooses a set SS of cells and Ivan tells him the parity of the number of black cells in SS . After knowing this, Peter is able to determine the parity of the number of adjacent\textit{adjacent} cells of different colours. Find all possible cardinalities of SS such that this is always possible independent of how Ivan chooses to colour the cells.

    Dataset text AI-MO AoPS. Quoted with source.

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