????????
      +---------
NXLZW | GZZNZHLU
       -MTRZV
        =====
        XJRHZH
       -XQPZGV
        ======
         XNRLHL
        -XLZJGW
         ======
          PRRQJU
         -PTWZUV
          ======
          ??????

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