????????
      +---------
NWYWG | QNLLYQJP
       -NWYWG
        =====
         PQGZQ
        -XNWZU
         =====
         NTVUGJ
        -ZUXYJW
         ======
          RQYPZP
         -RQWHGV
          ======
          ??????

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