carry down balance from previous row

Not sure I'm in the right sub-forum for my question but here goes.

I am creating a spreadsheet to measure my stock portfolio returns. In it I also wish to monitor the change in the number of shares held on the date of any given transaction.

For each row I have a beginning shares column, a shares bought or sold column, and an ending shares column.

I know how to use the SUM function to combine the values in the beginning shares column and the shares bought or sold column and to enter the result in the ending shares column. But I do not know how to carry down my ending shares column value to the next row to make it my new beginning shares value.

So, for example, if on row 10 my ending shares column value is 500 I want my beginning shares column value on row 11 to also be 500.

Apart from typing the new beginning number of shares can I get my row 10 ending share valuwe to carry down to ny row 11 beginning share value cell?