Replacing and adding data to columns with functions

Hello, my question is somewhat difficult to describe... so I'm sorry about the poor title.

Example:
Example.jpg

Let's say that we have a certain item and the sales on the item is being reported for each week. But in the second week, new items were added to the list in addition to the old ones. I want to add the new values sales to the week 2 column for all items that exist in week 1 and then add any items now in week 1 to the week 2 column. In a more algorithmic sense:

1. If the item name for week 2 = item name in week 1 >> add new value to week 2 column
2. If the item name for week 2 =/= item name in week 1 >> add item name to item name column and leave week 1 column blank

This is obviously easy to do manually in the example but I am dealing with > 500 items so I can't do it by hand. I am not familiar with all the Excel functions but if you give me the name of the function to use and general idea of the steps I can figure out the rest.

Please let me know if this was confusing and I'll try to find a different way to explain my problem.

Thank you!