????????
      +---------
6B7BF | 26F67839
       -76C61
        =====
        D1C178
       -B0A45F
        ======
         E36CE3
        -E82B3F
         ======
          BFE6C9
         -BCBAEF
          ======
          ??????

base: 16
letters: 0123456789ABCDEF
difficulty: hard