HNRZ
      +---------
HZZPP | GQQXZUHR
       -HZZPP
        =====
         QQMUU
        -UZMZM
         =====
         HQZUQH
        -HUZYWM
         ======
          HZVGNR
         -HJZPZT
          ======
           HVQWU

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging
missing: L