MDX Count on Filtered Set of Rows

I need to write an MDX query which goes through a filtered set of rows from my TEST_DW, and for each one it will return BelowZero=1 if MyValue<0, else BelowZero=0. I managed to complete this using the MDX below:

WITH 
MEMBER [Measures].[BelowZero] AS 
    case when ([Measures].[MyValue]) < 0
    then 1
    else 0
end 
SELECT NON EMPTY 
{ 
    [Measures].[BelowZero]
} ON COLUMNS, 
NON EMPTY 
{ 
    ([Accounts].[Number].ALLMEMBERS ) 
} 
ON ROWS FROM 
( 
    SELECT (STRTOMEMBER('[Date].[20130801]', CONSTRAINED) : STRTOMEMBER('[Date].[20130831]', CONSTRAINED)) ON COLUMNS 
    FROM [TEST_DW]
) 

The problem is that I now need to sum up all the values for BelowZero. In other words, this MDX should just give me one result back, telling me how many Accounts have Measure.MyValue<0. I am not interested in knowing WHICH accounts are <0, but just a count.

Any ideas please?