骑士动作 3

Place the numbers 1, 2,…, N (for some N) on a subset of the squares
above, so that it is possible for a knight to move from 1 to N via a
series of legal knight’s moves. Each number inside the grid represents
the height of a building located at that square, and we can think of
the knight as jumping from rooftop to rooftop on this series of
incrementally taller buildings.

A number outside the grid indicates the first (i.e. smallest) number
for which the knight was visible looking into the grid in the
direction of that row or column. (As shown in the example.)

The answer to this puzzle is the smallest achievable product of the
areas of the connected groups of empty squares in the completed grid.