????????
      +---------
NPGPP | RMPWLHZN
       -NPGPP
        =====
        QRQQNH
       -QNMWTZ
        ======
         YNJNZZ
        -YYTRTV
         ======
          YJVZVN
         -YRXVPJ
          ======
          ??????

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