WNRQ
      +---------
YFXQW | YPUYWJLQ
       -YFXQW
        =====
         UQPNJL
        -UFHFXR
         ======
          FPXLQQ
         -FUJNJQ
          ======
           UULYN

base: 13
letters: FHJLMNPQRUWXY
difficulty: medium
missing: M