SQL query issue

I am runnign this SQL query:

select

datepart(yy,archiveddate) as 'year',

datepart(mm,archiveddate) as 'month',

count(*) as 'items',

sum(itemsize)/1024/1024 as 'GB archived'

from saveset

where archiveddate > getdate() - 60

group by datepart(yy,archiveddate),

datepart(mm,archiveddate)

order by

datepart(yy,archiveddate) desc,

datepart(mm,archiveddate) desc

but getting this error : Msg 8115, Level 16, State 2, Line 1 Arithmetic overflow error converting expression to data type int.

iv read somethign about casting ? Can anyone advise here please?

Many Thanks

greg