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if u have wa 19
i use clear binmask as in 1122, but in 1122 u can chek for answer your board by every change of bit, BUT in THIS problem, u need check board when you finish your changes with bits.
Example u have bits 1101 and next is 1110,
u firstly change 1111 and then 1110 (two iterations because we need to do this iterations on board)
so if u do check on every that iteration its wa19