????????
      +---------
UNRXY | PUUWWPRG
       -UNRXY
        =====
        MNLGMP
       -MHWPHJ
        ======
         MNYGGR
        -MHWPHJ
         ======
          GRYHHG
         -GQNTJY
          ======
          ??????

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