i used this exporession, in the seconed column of slaes (column of expression) ,, it get it perfectly but it gets all values of the first dimonsion and it put '-' in the countries that doesn't start by 'a' ,
As I said, you won't need a calculated dimension. Change the calculated dimension back into a simple field name, e.g. contry.
Expressions have a default setting that 0 and NULL values should be suppressed. Leave that setting as is. Since your expression with set analysis will only produce results for countries that start with an 'a', all other countries will be suppressed.