????????
      +---------
MWPWZ | THXQTPYP
       -UYNUM
        =====
        NZQRUP
       -NUPJQY
        ======
         PVUPUY
        -PNGQHY
         ======
          NYZQGP
         -NMJNZR
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium
missing: L