????????
      +---------
XRURP | RYMTTNNN
       -RHXPX
        =====
         MXWZN
        -XRURP
         =====
          UGVYN
         -WYZYG
          =====
          XHUQXN
         -XLWZLU
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: hard
missing: J