WYHR
      +---------
QNNFR | UFUPYJFQ
       -QNNFR
        =====
        EXRNUJ
       -EWNHHG
        ======
         EWXFUF
        -FJGUXW
         ======
          QXHQWQ
         -XGWJQX
          ======
           QXUUW

base: 13
letters: EFGHJNPQRUWXY
difficulty: intermediate
restrictions: no multiplication