n=7 k=3
fence has more than 2n angles

You are devilishly right!
I figured it out when i drew a polygon of shape far from regular.
But it incredibly complicates the solution :((

Not really.If I understand your algo correctly,than it only needs to be improved a little.
Theoretically,my algo is O(n^3),but in tests my prog works less than 0,05 sec,because there're no such polygons,for which all the operations will be required.

Edited by author 06.03.2009 02:14