i found the right expression , it should be more like :

([dim_country].[hierarchy_Zone_setofcountry_country], [dim_branch].[hierarchy_branch_brand], [Measures].[Amount])/sum({[dim_country].[hierarchy_Zone_setofcountry_country].[All].children}, ([dim_branch].[hierarchy_branch_brand].[All], [Measures].[Amount]))

regards.