Macro to copy simple Sum down the collumn

Hi

I need a macro that can copy a simple formula

I have 2 columns one for the number of items and a column next to it that cumulatively add the totals down the sheet e.g. =E12+G13 etc.

A B
2 2 (=A1)
4 6 (=A1+B2)
1 7
3 10
2 12


If I copy the formula all the way down the page my graph becomes distorted
Is there a macro that will only copy the formula when there is data in the first column and run in the background without needing to be activated

Any help appreciated

Hi Thanks for your reply

The reason i dont want to drag the formula down is that i have about 400 lines that will need to be filled
If i drag it down, the graph still displays the figures for all the lines
I want the fromula to be pasted automatically into collumn B when a number is entered into collumn A
Is there a way of doing this?

Thanks for your help