If Cell A or B is not black then SUM

Hello everyone,

I am new to this forum and I severely need some help. I have been racking my brain trying to figure something out for this. I want Cell "C" to be blank unless Cell "A" OR "B" is not blank. This is what I have so far. At the moment "C" is blank unless both "A" AND "B" are not blank. Why is this acting like an "AND" function and not an "OR" function.

=IF(NOT(OR(B5="",B20="")),SUM(E4:E15),"")


A=B5
B=B20
C=current field