RPE
      +--------
NYFFR | GYEMXME
       -NYFFR
        =====
        PWPNGM
       -RLNPRP
        ======
         FJFERE
        -FPJELE
         ======
          YWXNW

base: 13
letters: EFGJLMNPQRWXY
difficulty: medium
missing: Q