Reaching Points

Run the moves backward

Forward moves replace either coordinate with its sum, so reverse moves subtract the smaller coordinate from the larger. Repeating subtraction one at a time is slow; when tx > ty, replace tx with tx % ty, unless ty is already the source boundary, where directly check whether (tx - sx) is divisible by ty. Mirror this for ty > tx. Stop when a coordinate falls below the source; success requires reaching exactly (sx, sy). The invariant is that reverse subtraction uniquely undoes the most recent forward addition while coordinates stay positive. Modulo batches many subtractions, giving roughly O(log max(tx, ty)) time and O(1) space. Test equal coordinates, source equals target, a coordinate already at the source, and targets that become zero after modulo.