????????
      +---------
GXFLJ | JMFWJMWL
       -GXFLJ
        =====
        YWPTUM
       -YRPWHW
        ======
         FUQNHW
        -QXNHPU
         ======
          XLJWWL
         -XGRXWJ
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard