Lcm hates Gcd

Chef has two integers $A$ and $B$. Chef wants to find the **minimum** value of $\texttt{lcm}(A, X) - \texttt{gcd}(B, X)$ where $X$ is any positive integer. Help him determine this value. Note: $\texttt{gcd}(P, Q)$ denotes the [greatest common divisor](https://en.wikipedia.org/wiki/Greatest_common_divisor) of $P$ and $Q$ and $\texttt{lcm}(P, Q)$ denotes the [least common multiple](https://en.wi