????????
      +---------
RVMVY | ZRYRMPXN
       -MRGRW
        =====
        LWGWMP
       -VJHTQY
        ======
         UXRGUX
        -UPZVXY
         ======
          VYURGN
         -VGVHUY
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: hard