Trying to connect two shapes at specific points and building a form between them.

Good morning,

I'm new to the community and hoping I could get some direction in solving my problem. As you will see from the .gh file, I am trying to create a stepping form and need a way to connect the two "risers" with a "tread" (to use stair language). I need to connect the bottom most point on the top riser to the top most point of the bottom riser.

I tried this with a polyline but was then having problems isolating the tread lines and discarding the riser lines. The intention is to then use the center point of the tread line to construct a pool (using the pool code and properly sizing the length based on the center point distance).

I'm also relatively new to grasshopper and could have made an error by way of grafting. I had it working when I teased out each individual riser but couldn't easily control quantity of risers with a number slider.

Thanks for your help!