Wikipedia on Delaunay Triangulation says:
"In the plane[...], if there are b vertices on the convex hull, then any triangulation of the points has at most 2n − 2 − b triangles, plus one exterior face."
Here n is the total number of vertices in the plane.
I can't see how they obtained this result when I use Euler's formula 1=V-E+F. So V=n then F=1-n+E. what is face exterior?