????????
      +---------
VPPLR | WRUPXNLN
       -VPPLR
        =====
         GRTNN
        -GMJNR
         =====
          XRHGL
         -MLLYZ
          =====
          TTJYWN
         -TTWVXZ
          ======
          ??????

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