Excel formula

In an Excel worksheet I have four rows of data. Row A show the months January – December for 2001 and Row B shows the expenditure in each month. Row E shows the months January – December for 2002 and Row F shows the expenditure for each month.

I need to show Year-To-Date expenditure for this year and compare it with the same month’s expenditure last year – and I want it to update automatically. When I fill in June’s spend, I want it to add the total spend Jan – June 2002 and display that total in one cell, then in an adjacent cell automatically display the corresponding total for the same group of months last year.

I know there must be a formula, but……
Any help would be appreciated

Thanks – Steve