Formual of conditional multiplication

Source: A1:B5

A1= 1 B1= 0
A2= 2 B2= 0
A3= 3 B3= 1
A4= 4 B4= 0
A5= 5 B5= 4

Question:
Looking for formula to multiply all value in A with 0 in B only
Per example, it will be 1*2*4
if I change the value in B5 to 0, it will be 1*2*4*5

thanks