I set up an arithmetic combinator exactly as in this video at 6:47. In the video, the combinator outputs the yellow with summed values. In my Factorio pictures, there is no yellow output.
I'm using Factorio version 1.1.19.
How do I get the arithmetic combinator (or other signal network pieces) to sum its inputs and output a single number? (What I actually want is to sum all the items on a belt, and activate an inserter if that sum is greater than some number.)