I am trying to come up with a formula that will accomplish the following.....

I have 2 columns that compare the current year weekly data to prior year's
weekly data. I need a formula that will sum Column D to same week in Column
B. Each week, the next row will be updated in Column B and the cells summed
in Column D need to change to include the next week's data too. So for the
table below, I would come up with the figure of 350 and the next week when B4
has data input, the total for Column D would be 500. I am trying to compare
total inventory each week. The cells in Column B with no data are blank.
Any ideas?
A B C
D
1 week ending 1-6-07 150 wek ending 1-7-06 200
2 week ending 1-13-07 100 week ending 1-14-06 100
3 week ending 1-20-07 75 week ending 1-21-06 50
4 week ending 1-27-07 week ending 1-28-06 150
5 week ending 2-3-07 week ending 2-4-06 225
6
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Sharon