????????
      +---------
MMPWM | LYRLJRFM
       -MMPWM
        =====
        QFQXTR
       -QPWMRW
        ======
         TWWPJF
        -YNWRPL
         ======
          YLFGLM
         -YNWRPL
          ======
          ??????

base: 13
letters: FGJLMNPQRTWXY
difficulty: hard