Convert string to number in formula for total sum.

Convert string to number in formula for total sum.

Hey all, thanks for the helpful response for my last question. Hopefully this one will be a just as painless. I'm trying to take the sum of 4 string values in order to calculate a total. I know in order to do this I need to convert those string values to numeric values however I seem to be missing something.
 
Tonumber(Sum1) + Tonumber(Sum2)
 
returns: Tonumber cannot be resolved.
 
I've also tried,
 
Sum1.toLong + Sum2.toLong
 
returns: Variable 'Sum1' is not declared
 
Thanks in advance!