It would be 0 because as you go along it doesn't go up but keeps moving. An undefined slope is up and down/vertical.
I believe the sum of the interior angles is the number of sides (n), subtracted by 2, and multiplied by 180.
(n - 2) x 180 = sum of interior angles of a n-gon
I also believe that the sum of exterior angles of a n-gon equals to 360 degrees.
