????????
      +---------
UZXUV | GPGRWXHJ
       -UZXUV
        =====
        MVLTXX
       -MGVJPP
        ======
         ZNQVVH
        - QTLNX
         ======
          ZGQGXJ
         -ZGRULJ
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium
missing: Y