The sum of the interior angles of any polygon is always . . .
(180 degrees ) times (2 less than the number of sides in the polygon) .
It doesn't matter whether the polygon is regular, haphazard, convex,
or all mangled and squashed etc. The only thing that matters is
how many sides it has.
The octagon has 8 sides. So the sum of interior angles is . . .
(180 degrees) times (8 - 2) = (180 x 6) = 1,080 degrees