Subtracting two cells if there are values in them

[josh from viauc]

i have been working on an excel sheet where i am collecting data from a house (how much electricity it uses)

I do not get a number every day, some some cells dont have number in them.



How can i make excel subtract the lower value from the top value.


There is a value in C4, but not in C5. So i want excel to subtract cell C6-C4?

Is there a way to make excel do this automactically?

Thank
Josh