Formula to compare previous year to current year

Looking for a formula to compare current year values to previous year values. For example, if the current year has values for the month of January through March (100, 100 and 150), current year value will be 350 and the previous year value will be 975 (i.e. 300+275+400). The aim here is to make the previous year months summation equal to the present (or current) values. As new values are entered for the current year, the previous year's values will have to change to reflect the new month's value entered for the current year.

Month 2012 2013
Jan 300 100
Feb 275 100
Mar 400 150
April 650
May 454
June 800
July 500
Aug 375
Sep 525
Oct. 300
Nov 410
Dec 510

Sample file is attached for your convenience.

Thanks for the assistance.

Alkey: Thanks for the response. If you had a new value for the month of April, 2013, the proposed value should give the total of 450 (January through April 2013) and 1625 for January through April for 2012. However, your suggested formula is not expanding as new values are added to the current year (months).