????????
      +---------
NWGHJ | GJYHGTNH
       -NWGHJ
        =====
        UTTQPT
       -UTHMGR
        ======
          HUTRN
         -NWGHJ
          =====
          JJUPUH
         - RPRWU
          ======
          ??????

base: 13
letters: FGHJMNPQRTUWY
difficulty: medium
missing: F