I want to use SUMPROCUT function on condition that the sheet's row count is unknown.

So the formula was written by =SUMPRODUCT(--F:F,--G:G), after Enter key be pressed #VALUE! showed up.

I realized that columns name are string type but they were included in calculation, that could be the problem.
How to avoid String text in SUMPRODUCT function? Maybe you have a smarter way?

Clients asked me a formula to calculate sum(column F * column G), such as 10*470 + 5*350 + 5*5.5 + 25*180 + 4*48 + 6*15 + 80*4.5 + 70*5.44, and I don't know how many rows in their sheet caused. They want a general formula to deal with all excel files.