????????
      +---------
KWWJK | KHNNKPCP
       -KWWJK
        =====
         HNGWP
        -HWPCH
         =====
          BBHHC
         -BWPBB
          =====
           BCBPP
          -BWPBB
           =====
           ?????

base: 10
letters: ABCGHJKNPW
difficulty: hard
missing: A