Adding a default entry or "-" in a cell, based on another column being empty.

I'm trying to create a costings table where you can specifiy the poduct code (thus price per metre), it's length and a range of other variables which affect the price.

Some of the products we sell need to have a length specified per piece, others are just parts you need to specify a quantity of.

My basic table is as follows (cell numbers shown):

A B
11 Qty. Length
12
13
14

What I'm wanting to achieve is:

If 'Qty'. is blank, then 'Length' is blank by default
If 'Qty.' shows a number then 'Length' shows '-' by default

'-' should be able to be manually over-ridden with a typed in number.

Can anyone point me in the right direction?

=IF(A12="",B12=""), IF(A12="??????",B12="-")

I'm pretty new to excel functions so am I on the right track, what should ????? be in the equation above.

Many thanks in advance for any help!

Both work thanks guys. Always numerous ways to get the same result, thanks for the speedy replies.