Multiple Attribute Values in Calculated Member

Hi

I have very little experience in MDX and I'm struggling with creating a calculated member in SSRS. I was hoping someone could help me please.

The scenario....

London 3

Oxford 4

Bournemouth 5

I would only like to sum London and Oxford and not Bournemouth. Also, I only want to see the measure value.

What I have....

({[Dimension].[Attribute].&[London],[Dimension].[Attribute].&[Oxford]},[Measures].[Nr Of Visits])

This returns NULL.

Please could someone let me know how to return the figure 7.

Thanks for your help

Lucy