????????
      +---------
YTHLJ | XYRFLGGN
       -YTHLJ
        =====
        PLQPWG
       -PURQHM
        ======
         YWJQWG
        -YGYHHN
         ======
          HLJGHN
         -HPNYXR
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: challenging