????????
      +---------
PNMNJ | PRFLUMTH
       -PNMNJ
        =====
         PJPWM
        -PNMNJ
         =====
          LTNPT
         -NXGXM
          =====
           QRRHH
          -QFJPG
           =====
           ?????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard
missing: Y