Creating a loop with a small formula

Hi,

Im trying to get the following thing started but my loop doesn't succeed. I've got 3 columns. I need to place a formula in column C if A is empty.

Example:

A2 contains data so do nothing.
A3 is empty do: C3=B3+B4.
A4 contains data so do nothing.
A5 contains data so do nothing.
A6 contains data so do nothing.
A7 is empty do: C7=B7+B8.

this is what I've got so far:

Dim rCell As Range

    If IsEmpty(rCell) Then
        ActiveCell.Offset(0, 3).Select
        ActiveCell.FormulaR1C1 = "=R[-1]C[-1]-RC[-1]"
    Else

    End If

If you need an examplefile, just say so : )