????????
      +---------
LPULJ | HZHPUJXV
       -WHPWR
        =====
        LVPNGJ
       -LYMHLH
        ======
          QPXRX
         -NVXNG
          =====
          VVTTGV
         -VTRWVW
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging