Validation rule - when the Billing Country is equal a certain value the Account Billing State/Province is a required manatadory field

Like this = 

AND(
    CASE(
        BillingCountry,
        "United States",    1,
        "Canada",           1,
        "Austraila",        1,
        "Brazil",           1,
        "China",            1,
        "India",            1,
        "Ireland",          1,
        "Italy",            1,
        "Mexico",           1,
        0
    ) = 1,
    ISBLANK(BillingState)
)