SUM Distinct Measure

Hi,

I Have a fact table which looks like:

I want to sum up the distinct values? So rather than the sum being 60950 it needs to be 34300.

I was thinking on the lines:

SUM(
(
NONEMPTY([Customer].[Customer Number].[Customer Number].members,
[Year Month].[Year Month].[Year Month].Members)

,[Measures].[Available]
)

Any ideas?

Thanks