????????
      +---------
MWGJH | QHLNMQQQ
       -MWGJH
        =====
        PMHWUQ
       -PWHUUX
        ======
         NYQXQQ
        -NJHNTH
         ======
           UJPRQ
          -MWGJH
           =====
           ?????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: medium
missing: F