????????
      +---------
LNFPJ | MGFYXYUY
       -LNFPJ
        =====
        HNPYFY
       -HQPTPM
        ======
         NQUYLU
        -WRXQJJ
         ======
          THFXYY
         -HYUGPQ
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard