jmc

For each positive integer $n$, the set of integers $\{0,1,\dots ,n-1\}$ is known as the $\text{residue system modulo}$ $n$. Within the residue system modulo $2^4$, let $A$ be the sum of all invertible integers modulo $2^4$ and let $B$ be the sum all of non-invertible integers modulo $2^4$. What is $A-B$?