Function that follow collums

Heya bois i have a simple problem that i have not been able to find an answer to with my lack of keyword knowledge. 

I have this function =B9*B13*B14+C9*C13*C14+...

and i want it to continue to follow the letters up x amount of times, without having to do a lot of clicking!

how do i make that happen? please help thanks uwu!