?????????
      +----------
73B66 | 77B47A219
       -7890BA
        ======
         71C282
        -7A86B0
         ======
          981811
         -C164B1
          ======
           777389
          -7890BA
           ======
           ??????

base: 13
letters: 0123456789ABC
missing: 5
difficulty: intermediate