????????
      +---------
WRYHT | HLHGPUJJ
       -WRYHT
        =====
         PRRQU
        -WRYHT
         =====
         WXHYFJ
        -GRFQPX
         ======
          NHULJJ
         -NGRLUT
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: medium
hints: U << M
missing: M