Excel Formula

[Sandycheeks]

Can any of you kind folk provide a formula for the following, please?

B19 = B8-B17
If no data has been entered into B8 I want the sum to use B5 instead.

I know this might seem really simple but I just don't have much experience with excel.

 

[Sandycheeks]

Wow thanks for the speedy response but it doesn't seem to work.
I think my explanation was off when I stated "if no data entered in B8".
B8 is a sum from elsewhere that could be 0.

 

[Sandycheeks]

Perfect!
Thanks very much.