Using one of the rectangular faces as a base for the next pyramid, there would be the addition of 1 vertex, 4 edges, and 4 faces.
Using one of the triangular faces as a base, there would be the addition of 1 vertex, 3 edges, and 3 faces.
Therefore for a minimal sum, one of the triangular faces must be the base of the pyramid.
The existing figure has 6 vertices, 9 edges, and 5 faces.
6+9+5 = 20
With the addition of the pyramid, there would be an extra 1 vertex, 3 edges, and 3 faces.
1+3+3 = 7
But note that the base of the pyramid would no longer be a face - so we subtract one from our total:
20+7-1= 26.
Thus, the minimal sum is 26.