????????
      +---------
NRGPR | MPGLWGUL
       -NRGPR
        =====
        PMHMNG
       -PUGQJN
        ======
         FRQFPU
        -QMUULM
         ======
          XJJJFL
         -XHWGPQ
          ======
          ??????

base: 13
letters: FGHJLMNPQRUWX
difficulty: hard