Without using Calculus I can't prove its the shortest path, but what you need to do is walk a straight line from the SW corner of the lot to the SE corner of the building, then to the NE corner of the lot. Alternately you could go from the SW corner of the lot to the NW corner of the building, then to the NE corner of the lot.

Since all the paths are straight lines you can use the Pythagorean Theorem to figure out each distance.

-Dan