Lead/lag series

Hi I am planning to estimate forecast of revenue of my project. I am having data of last few years. I want to generate a new column which contains the revenue lag. Example is in Revenue lag column the value of February is equal to the value of January in revenue column, Value of march in revenue lag column is equal to value of revenue in february column.

Month Product Revenue Revenue Lag
------- --------- -------------- -------------
Jan2012 A 10
Feb2012 B 13
Mar2012 C 23
Apr2012 D 14
May2012 E 45
Jan2012 A 12 10
Feb2012 B 16 13
Mar2012 C 12 23
Apr2012 D 23 14
May2012 E 12 45
Jan2013 A 67 12
Feb2013 B 78 16
Mar2013 C 45 12
Apr2013 D 34 23