????????
      +---------
NNHLJ | JYHMHUMQ
       -NNHLJ
        =====
        MHUPWU
       -MXMJWP
        ======
         XLJFGM
        -XHPHTQ
         ======
          NTGTWQ
         -RUGRGQ
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard