????????
      +---------
NHQJT | XHNMGRYM
       -VYJWU
        =====
        NMGPPR
       -NMMZZJ
        ======
          UTGYY
         -NHQJT
          =====
          YLVTRM
         - XGPGR
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium