This circuit contains 4 inputs, 2 of each inputs are connected to an adder. The top inputs are 1's, and the lower inputs are 2's. After going through the adder(s), the output connects to the decoder input (click the tab "Decoder"). The decoder is responsible for taking binary inputs and decoding into a display that you can read. The decoder is set up to return the value "7," but I found that harder than it looked...