Excel Formulas

I have four rows of data on an Excel sheet. Row 1 contains months for the year 2001. Row 2 contains expenditure for each month.
Row 4 contains months for 2002 and row 5 contains expenditure for each month.

I need to show Year-To-Date figures forboth years but to update them automatically. When I input the total spend for June, I want one cell to display the total spend for the year 2002, January – June. I then want an adjoining cell to automatically display the January – June figure for 2001.

When I enter the July 2002 spend, I want the 2001 total to update automatically.

I have looked at all sorts of info about array constants, but cannot find one that will do what I want.

Any help would be appreciated.

Thanks – Steve