Put No. in Offset cell when value is entered
 

Hi all, How should I write macro code if I need no. to popup in
offset cell of column B cell if I enter any value in it. Like I want
macro to put 1 in cell A1 if I put any value in cell B1 and 2 in cell
A2 if I put any value in cell B2 and (3,4,5...)carry on down to row
500. I know the formula to do it but i want to do it by macro.
Please can any friend can help and please also advise what changes
could be make in macro code which should cover if i leave any cell
blank in column B in between when putting the values in cell.