assign BIGINIT value to some variable

Hi,
I want to assign whatever result ABS(CAST(CAST(NEWID() AS VARBINARY(5)) AS Bigint)) this expression gives to some variable.
I am using Vb language.
I tried
Dim a 
a=ABS(CAST(CAST(NEWID() AS VARBINARY(5)) AS Bigint))
It gives error.
How can i do so?

...