Ok, I need help. I took two formulas and tried to combine them to make I sort of If-Then formula. If LEFT is true, then count the amount to the right. Please help!

=IF(([Orderfilling.xlsx]LEFT(Sheet1!K5:K50),"CP"=TRUE, SUM([Orderfilling.xlsx]Sheet1!L5:!L50))



Edit: Actually, this formula is probably close to what I'm trying to do, but still flawed;
=IF(ISNA([Orderfilling.xlsx]Sheet1!K5:K50,"CP"=TRUE, SUM([Orderfilling.xlsx]Sheet1!L5:!L50))