mondrian aggregate the last value

hi,
we have some problems in using mondrian.
aggregate the last value.


we have the following dataset:
Month Payment
201201 2000
201202 1500
... ...
201211 1800
201212 1200
the according mdx statement:
select {[Time].[2012].children} on axis(1), {[Measures].[Payment]} on axis(0) from SCHEMA_A


after Aggregate, we calculate the payment of year 2012,
we want to get the value 1200, also the value of month 201212, not the sum value 2000+1500+...+1800+1200.
Year Payment
2012 1200
select {[Time].[2012]} on axis(1), {[Measures].[Payment]} on axis(0) from SCHEMA_A


and, we do not want to use the mdx to calculate it, we use the mdx to get just the result only.
we just want to use the mondrian schema, such as use the attribute or the CalculatedMember, we think it ourselves like that, but we didn't do it.
so, how to do that?


would you please give some advice?
thanks a lot.