Two Cells -> One Cell Accumulator

I have this code:

If Not Application.Intersect(Target, Range("C6:C999")) Is Nothing Then
Application.EnableEvents = False
Target.Offset(0, 1).Value = Target.Offset(0, 1).Value + Target.Value 'C to D: 1
End If

As you can see, when I enter a number into column C it accumulates onto column D.

I want to be able to remove column C altogether so that when I enter a number into any cell in column D it accumulates there.

For example, D45=324, I enter 218 into D45, press enter, D45 now reads 542.

This only changes D45 and no other cell, but applies to all cells from D6 downwards.