A convex 1993-gon is divided into convex 7-gons. Prove that there are 3 neighbouring sides of the 1993-gon belonging to one such 7-gon. (A vertex of a 7-gon may not be positioned on the interior of a side of the 1993-gon, and two 7-gons either have no common points, exactly one common vertex or a complete common side.)(A Kanel-Belov) combinatoricsgeometrycombinatorial geometry