Selecting and restraining nodes out of a list

Hello everyone, I am currently trying to analyse a structural model in GSA. I face  some difficulties though, when trying to isolate the nodes that are on the ground (or z = 0) from the list of the nodes that are created when I use the mesh-to-finite-element tab. I upload a very simple model in order to help you better understand what is my actual question. I should also note that in the model that I am going to use eventually, the number of the nodes at the base will have to change, so I have to come with with an automated solution. Thanks in advance