????????
      +---------
RQFUR | MPFPNYFT
       -RQFUR
        =====
        UQJHXY
       -UMYPWU
        ======
         UJTHTF
        - YUHQP
         ======
          UPJQPT
         -UWPTPQ
          ======
          ??????

base: 13
letters: FHJMNPQRTUWXY
difficulty: intermediate