When do we write a hyphen in adverbs?

We write a hyphen in adverbs when they start with по- and finish with -и. For example when we want to say "In Russian" (по-русски) we should use a hyphen. There is an other case when se must use a hyphen in adverbs. It is when they start with во- or в-. This rule is useful when we have to write such expressions as "firstly" (во-первых), "secondly" (во-вторых) and so on.