????????
      +---------
UNNJY | WTVRWZZJ
       -UNNJY
        =====
        PPLMNZ
       -YWYPWM
        ======
         UPQJMZ
        -UVQYMJ
         ======
          YQNHJJ
         -RXQVVH
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: hard
missing: G