17
4
abcd
    aaaaaaaaaa
    a bbbbb  a
    a b   b  a
    a b   b  a
   cccccccccccc
   ca b   b  ac
   caabaaabaaac
   c  b   b   c
   c  b   b   c
   c  b   b   c
   c  b   b   ddd
   c  bbbbb   d d
   c          d d
   cccccccccccd d
              ddd