?????????
      +----------
RVMUY | VGNTNGRJL
       -RVMUY
        =====
        MNGNHG
       -HWYHGP
        ======
         UXYYGR
        -UVTLNV
         ======
          YUMTTJ
         -GXGGQQ
          ======
           RHJHYL
          -UVTLNV
           ======
           ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: intermediate
missing: Z