We call a pair $(a,b)$ of positive integers, $a<391$, [i]pupusa[/i] if 

$$\textup{lcm}(a,b)>\textup{lcm}(a,391)$$

Find the minimum value of $b$ across all [i]pupusa[/i] pairs.

Fun Fact: OMCC 2017 was held in El Salvador. [i]Pupusa[/i] is their national dish. It is a corn tortilla filled with cheese, meat, etc.