Excel 2007 - Comparison of monthly sales year on year

Hi,

Can anyone help please?

I have a spreadsheet that shows sales figures from 2011/12 and 2012/13. What I need to do is to keep a running total of whether the total sales YTD are better or worse than the figures at this time last year. For example, four monthly sales figures for 2012/13 to date are:
1897, 2079,1395 and 662.
I am inputting these figures at the end of every month.

I also have all the monthly sales figures for the whole of last year but the only four I am interested in is the four from the corresponding months last year which are:
1478, 3601, 3937 and 1923

I would also like it that when I input the next months figures it automatically calculates the increase / decrease of total sales from the corresponding 5 months of last year.

I hope this makes sense - I have attached a copy of the spreadsheet to try and make it a bit easier to understand. It is Column R I need a formula for.

Thanks in advance

EXAMPLE.xls