Selecting faces in buildings

Grasshopper
1

Hey,
so I have 95 buildings in grasshopper, and I want to work on them, but only in their top faces. as usual, I would use list item, and select just the top faces, but each buidling has different number of faces, so I can’t get all the top faces in once, and I don’t want to select each one separately.
what can I do?

thanks!

3

thanks a lot!!! works perfectly!
But just for understanding, you mind to explain the logic behind it?

5

so simple.
thx again