????????
      +---------
GFQFP | QXEJJHUE
       -GFQFP
        =====
         JNPFH
        -UFMFH
         =====
         XXPMMU
        -XMWRMJ
         ======
           PNWWE
          -FEPEW
           =====
           ?????

base: 13
letters: EFGHJMNPQRUWX
difficulty: medium