????????
      +---------
QUWJF | MXEXNMNN
       -QUWJF
        =====
        WFJUQM
       - QUWJF
        ======
         UQENPN
        -UGUXJM
         ======
          UMGHPN
         -UHPPGJ
          ======
          ??????

base: 13
letters: EFGHJLMNPQUWX
difficulty: easy
missing: L