FRR
      +--------
UYFXX | XJMRJTT
       -UYFXX
        =====
        MQNWTT
       -MXMYTY
        ======
         MQFNPT
        -MXMYTY
         ======
          MQPEP

base: 13
letters: EFJMNPQRTUWXY
difficulty: hard
missing: E