Multiply and sum two columns based on criteria?

[Ronix]

Hi,

I need to multiply and then sum in Excel 2013 two columns D and K based on the name "BOOK" which is a part of the string in some rows of column C.
I´ve tried this formula below but no avail.
What´s wrong there?

=SUMPRODUCT(D6:D50;K6:K50*(C6:C50="*BOOK*"))

Thx in advance!

 

[njimack]

=sumproduct(--(ISNUMBER(SEARCH("BOOK",C6:C50))),(D6:D50*K6:K50))

 

[Ronix]

=sumproduct(--(ISNUMBER(SEARCH("BOOK",C6:C50))),(D6:D50*K6:K50))

Thx a lot, it´s working perfectly.
What is double dash (--) purpose in that formula please?