?????????
      +----------
QWYMU | PHGGQRTVM
       -QWYMU
        =====
        GMHUZR
       -GHWZPM
        ======
         GMPRWT
        -GHWZPM
         ======
          GTHTVV
         -GTXJRJ
          ======
            RZLQM
           -THWJJ
            =====
            ?????

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