????????
      +---------
HNVQU | HQRZPQLW
       -HNVQU
        =====
         PJXMQ
        -HNVQU
         =====
         JLHYJL
        -JQGQMU
         ======
          TPRNYW
         -TWWYVU
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging