IF column B = closed, then calculate # of days between dates in Column C and Column D

I have a worksheet and within it it has column B which states if a deal is closed or not (Closed is typed in if closed and the cell remains empty if the matter is not closed). Then I have column C with opening date and column D with closing date. I need a formula that will allow me to say if B1 is "Closed" then calculate the number of days difference between column C and column D.

I think I could use:

=IF(B1="Closed",(=DATEDIF(c1,d1,"D")),"")

But it does not work. Any ideas ? Am I trying to mix too many formulas. Any input would be greatly appreciated.

Thank you