Change only a part of formula when drag drop in Excel

So I was just doing a HEX to RGB converter in excel, but when I pull it to the other cells, there's a part of the formula that I didn't wan't it to change.

Example:

=GetRGBFromHex(B2,C1)

Bold=don't want to change

But Excel decided to change it anyway, and I have absolutely no idea how to fix this...

Any help will be appreciated. Thanks!