For each positive integer n , the set of integers is known as the n . Within the residue system modulo , let A be the

For each positive integer n , the set of integers $\{0,1,\ldots,n-1\}$  is known as the $\textit{residue system modulo}$  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?