Price Changes

I have to make 'New Price' based on the 'Period' changes. For example if the number in 'Period' is 3 that means the 'New Price' for Jan and feb '13 are exactly the same as 'Market Price', but on March 13 is the average price of the previous two months. Important note: If the number in 'Period' is changed to other number, I need to make sure that the 'New Price' will automatically changed. For example if the 'Period' is 4 that means, the first four months of 'New Price' will be the same as the 'Market Price' but the in the fifth month will be the average price of the previous four. Please help.
Exercise.xlsx