I am making a budget spreadsheet in which I keep each month's expenses
in one column and then the annual expenses in another column.

For example, the January (month 1) expenses for line item 1001 are
$100 and therefore the annual expenses for line item 1001 are $100.

In February the 1001 expenses are $200 and therefore the annual 1001
expenses are last month's 1001 expenses ($100) plus this month's 1001
expenses ($200) which is $300.

Without keeping a separate column for each month's line item expenses,
is there a tricky way of adding a number to itself? That is, when I
enter $200 into the February spreadsheet how can I add the annual
expenses to it without keeping a separate tally sheet somewhere else?

Jim