Do I use an IF function?

Here is what I'm trying to do. I'm making an expense
record chart. I am entering a negative (-) number for
items paid with a credit card and a positve (+) number
for items paid with cash. Hence, I will have both
negative AND positive numbers in the same column (I
cannot use two separate columns, for lack of space). I
would like to know a formula or use an IF function that
will total up the negative (-) numbers only. Does anyone
know how to do this, or if it is even possible? Thanks!