If there are multiple (X, Y); choose one with the smallest X.
A = 1
A = 2
there is only one possible pair to choose, (1,1)
There are four options: (1,1), (1,2), (2,1), (2,2). (1,1) and (2,2) have xor 0 whereas (1,2) and (2,1) have xor 3. We chose (1,2) since it has smaller x.
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