How to Use R1C1 format

VBA  macro code to calculate average in a spreadsheet :



Sub calcaverage()
Range("D1").Select
ActiveCell.Value = "Average"
ActiveCell.Offset(1, 0).Select
    Do While Not IsEmpty(ActiveCell.Offset(0, -2))
        ActiveCell.FormulaR1C1 = "=Average(RC[-3],RC[-2])"
        ActiveCell.Offset(1, 0).Select
    Loop
End Sub