????????
      +---------
LMJRY | QRJUHRHF
       -JFHFJ
        =====
        LJJXMR
       -LRURFP
        ======
          HRPXH
         -FQJEU
          =====
           YUXXF
          -PYHPM
           =====
           ?????

base: 13
letters: EFHJLMNPQRUXY
difficulty: intermediate
missing: N