Premium Letters : B , C , D , F , G , H , L , M , N , P , R , S , T
Vowels : A , E , I , O , ( U - not considering it )
Acc to my opinion , U is the least favorable vowel so not included in my list.
So 13 premium letters and 4 vowels , total combination possible is 13 x 4 x 13 x 4 = 2704...