????????
      +---------
EHTXP | PJPMHJFN
       -PTXHY
        =====
         QTPPJ
        -PTXHY
         =====
          MQNQF
         -MERUF
          =====
           PTQUN
          -EHTXP
           =====
           ?????

base: 13
letters: EFHJMNPQRTUXY
difficulty: medium