Let $n$ be a positive integer greater than or equal to $3$. Let $a,b$ be integers such that $ab$ is invertible modulo $n$ and $(ab)^{-1}\equiv 2\pmod n$. Given $a b$ is invertible, what is the remainder when $(a b)^{-1}(a^{-1} b^{-1})$ is divided by $n$