Setting a specific number to a cell where the other numbers in the columns will substract

First of all, I know nothing about Excel and this is pretty much the first time I am using it.

How can I set a rule about this?

I set the number 12 to a column for instance.

Then I write 5 to the second row of the same column.

I want 5 to be subtracted from 12 then the value will be shown in the sum row (Not in the same row.)

Like this:

B Column = 12

B Column, 1st Row = 5 (12-5 = 7)

B Column, 2nd Row = 15 (12-15 = -3)

B Column Sum Row = 4 (7 -3)

Thanks in advance. Sorry for my English.

I found the solution with a help with adding this formula:

=number*COUNT(B:B)-SUM(B:B)

Problem solved

Removed by FR.