????????
      +---------
WNFQH | FQHWJQNL
       -WNFQH
        =====
        WHGTGQ
       -WPQLWN
        ======
          GRFYN
         -UGMMY
          =====
          YHUTJL
         -YPGJUL
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard
missing: X