A switching network has $3$ inputs and $2$ outputs. The output variables $a_1$ and $a_0$ represent the first and second bits, respectively, of a binary number, $N.N$ is the number of inputs $(w,x,y)$ which are $"1".$ For example, if $w=1,x=0,y=1$ then there are two inputs which are $"0",$ so $a_1=1$ and $a_0=0.$