YTD Total Minus Previous Month

I can't seem to wrap my head around how to accomplish this. I have a year-to-date running total with all the months listed. It keeps a running total anytime a new number is added. What I would like to do is have a separate cell that gives me the running total minus the previous month and would update automatically each time a new value is entered.

This is how it should look with Feb being filled out at the end of the month so I would then want a previous month cell to give me the totals of Nov-13 - Jan 14 and my YTD will give me the totals of Nov-13 - Feb-14.
Month Monthly Total
Nov-13 136676
Dec-13 140085
Jan-14 122142
Feb-14 0
Mar-14 0
Apr-14 0
May-14 0
Jun-14 0
Jul-14 0
Aug-14 0
Sep-14 0
Oct-14 0
Nov-14 0
Dec-14 0
YTD 122142

It seems like it should be simple but I just can't figure out how to accomplish it or maybe its not a possible function.

This works until more information is added. If numbers are added for February, it gives the new total, but the previous month doesn't update. Is this possible to do?