?????????
      +----------
FC50E | 2F2F4FF9F
       -FC50E
        =====
        A03CFF
       -A583BE
        ======
         A2374F
        -A372D7
         ======
          5EBAF9
         -50B67E
          ======
           C44FAF
          -C2DE57
           ======
           ??????

base: 16
letters: 0123456789ABCDEF
difficulty: intermediate
missing: 1