RGL
      +---------
LUFXF | PURWEMYW
       -UFWWEN
        ======
         RPQQUY
        -EQQRMQ
         ======
          EMLWLW
         -PNYGGR
          ======
           UMEFG

base: 13
letters: EFGLMNPQRUWXY
difficulty: medium
restrictions: no multiplication