The number of triangles whose vertices are at the vertices of an octagon but none of whose sides happen to come from the sides of the octagon is:

The number of triangles whose vertices are a the vertices of an octagon but none of whose sides happen to come from the sides of the octagon is

The number of triangle whose vertices are at the vertices of an octagon but none of whose sides happen to come from the sides of the octagon is

The number of triangles whose vertices are a the vertices of an octagon but none of whose sides happen to come from the sides of the octagon is

Find the number of triangles whose angular points are at the angular points of a polygon of 13 sides, but none of whose sides are the sides of the polygon.