????????
      +---------
XLQZX | YLXTRVYJ
       -ZXVTL
        =====
        XHTPQV
       -XWZJHU
        ======
         WVXWWY
        -WUNTTG
         ======
           LNUWJ
          -QUGJQ
           =====
           ?????

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