????????
      +---------
NXMFG | RQQNJJNG
       -RNYPJ
        =====
         WYJHJ
        -GRPJM
         =====
         XWFXWN
        -XMJYYN
         ======
          XFNLHG
         -XFXRGF
          ======
          ??????

base: 13
letters: FGHJLMNPQRWXY
difficulty: hard