????????
      +---------
PRGRF | NLLWJQPQ
       -NTFTY
        =====
         HWFJQ
        -HPQYF
         =====
          MPXLP
         -PRGRF
          =====
           TGHHQ
          -RQRTU
           =====
           ?????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard