The sum of the exterior angles of any regular polygon is 360°. The sum of the interior angles is (N-2)180°, where N is the number of sides.

Because the exterior angles are 90°, we know the polygon has (360/90 = 4) sides. So, the interior angles add to (4-2)180° = 360°

Because the exterior angles are 90°, we know the polygon has (360/90 = 4) sides. So, the interior angles add to (4-2)180° = 360°