Is there a way to fill a column down which contains a calculation without
having the result appear until the actual data is inputted?

For a simple example: Column C is a running total of A and B, with Cs
formula filled down to Row 5. I dont want to have the running total appear
in Column C for rows that havent any data entered. As shown below, I dont
want the last three 6s appearing. The formula, beginning at C2 is:
C2 =SUM(A2:B2)+C1
C3 =SUM(A3:B3)+C2
etc.

Thank you for your assistance.

A B C
1 2 3
1 2 6
6
6
6