Following from yesterday's exploits I've now come across a similar problem but this time involving drop down menus

I have two columns A and B

A - states the quantity
B - states the type

If 'A' = '-' from the pull down list (representing zero / not applicable) I want column B to show '-' automatically.

If 'A' shows some other option from the pull down list I want to be able to manually select the type from the pull down list for 'B'

Both lists have '-' as a manually selectable option from the pull down list if that helps.

When I say pull down list I'm referring to Data>Validation tool and selection 'List' and specifying a range.

I'm having trouble creating a formula condition for column B as the list seems to restrict the content and prevent formulas?

Hope someone can point me in the right direction again! Thanks