MWRT
      +---------
YXYHM | LJWHTMRF
       -YXYHM
        =====
        HWUPHM
       -MGRQNW
        ======
         FMMNUR
        -WGWJFR
         ======
          MJUYPF
         -MYWXXT
          ======
           WNPLH

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: medium