Sumif Function

Trying to solve an issue with the following scenario:

Cells A1-A4 Sum into Cell A5

and Cell A6 has a value in it (Such as 100)

Then the result of the logic to be as such:

If Cell A5 is Not Zero, then take the value of cell A6-A5 and sum into A7
If Cell A5 is Zero, and A6 has the same value (100) place a Zero into A7

A1 10
A2 5
A3 0
A4 5
A5 20
A6 100
A7 80

or

A1 0
A2 0
A3 0
A4 0
A5 0
A6 100
A7 0

TIA