UMM
      +--------
HPNGQ | WNXQNPU
       -TLHNF
        =====
        NMLJJP
       -NJMTFR
        ======
         NMLTPU
        -NJMTFR
         ======
          NQRNU

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard
missing: Y