????????
      +---------
PNEEW | PHNHUYXJ
       -PNEEW
        =====
         XJFJY
        -XRFFJ
         =====
          XFUUX
         -PNEEW
          =====
           RNMMJ
          -YPFXH
           =====
           ?????

base: 13
letters: EFHJLMNPRUWXY
difficulty: medium
missing: L