Unique count to date - Help

Hello, can anybody indicate the MDX to calculate Unique Count to date ?

Having two Periods

Period Unique Count Unique Count to Date
First 10 10
Second 4 Unique Count (First + Second)


So if 2 of the 4 items are contained in the first period, then the unique
count should be 10 + 2 = 12

Thanks in advance,

Alex