ssrs sum based upon a conditional statement

In an ssrs 2008 r2 report, I have the following code that totals a transaction amount:
=sum(cdec(Fields!TransactionAmount.Value))

Now I need to have different total amounts based upon 'payment type'. The payment_types are either 'check', or 'credit' for credit card. Thus can you show me how to change the code I just listed to sum the amount depending upon the payment type?