We’re at the Polygons annual party. It’s polygons only so to get a ticket, you have to be a 2D shape, with straight sides. Polygons can be grouped based the number of sides they have.

A polygon is a closed 2D shape with at least three sides. A regular polygon is a polygon ... and then multiply this by 180°. This gives 8 × 180 = 1440. The interior angles in a decagon sum ...