????????
      +---------
NJJFF | XRNJTTRY
       -NJJFF
        =====
        GYXMGT
       -PRTNNF
        ======
         PYTNGR
        - NJJFF
         ======
          MXTTWY
         -MNMPXF
          ======
          ??????

base: 12
letters: FGJLMNPRTWXY
missing: L
difficulty: medium