get the previous last value with SSRS

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Hi,
I create a report with SSRS 2008 in this format :
year
Data 2010 2011 2012
hp 14 25 30
Dell 17 18 20

what i need when i create the report i want to have another column with the last value concatenation with the last minus or previous one like this :

year
Data 2010 2011 2012 2012/2011
hp 14 25 30 3025
Dell 17 18 20 2018

i tried the last function i got the last value with this function:

=Last(Field!Qte.value) I got the right result 30 and 20

and i tried also the last minus or previous one with this code

=Last(Field!Qte.value)-1 I got the false result 29 and 18