????????
      +---------
FNMML | JRGPGFGX
       -FNMML
        =====
         XUWFF
        -FNMML
         =====
         TYMXWG
        - HJQPM
         ======
          FXGYTX
         -FRFTGM
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard