Problem:
Problem B: Red and Black

There is a rectangular room, covered with square tiles. Each tile is
colored either red or black. A man is standing on a black tile.
From a tile, he can move to one of four adjacent tiles. But he can't
move on red tiles, he can move only on black tiles.

Write a program to count the number of black tiles which he can reach
by repeating the moves described above.

Input

The input consists of multiple data sets.
A data set starts with a line containing two positive integers W and H;
W and H are the numbers of tiles in the x-
and y- directions, respectively. W and H are
not more than 20.

There are H more lines in the data set, each of which
includes W characters. Each character represents the color of a
tile as follows.

'.' - a black tile

'#' - a red tile

'@' - a man on a black tile(appears exactly once in a data set)

The end of the input is indicated by a line consisting of two zeros.

Output

For each data set, your program
should output a line which contains the number of
tiles he can reach from the initial tile (including itself).

Sample Input

6 9
....#.
.....#
......
......
......
......
......
#@...#
.#..#.
11 9
.#.........
.#.#######.
.#.#.....#.
.#.#.###.#.
.#.#..@#.#.
.#.#####.#.
.#.......#.
.#########.
...........
11 6
..#..#..#..
..#..#..#..
..#..#..###
..#..#..#@.
..#..#..#..
..#..#..#..
7 7
..#.#..
..#.#..
###.###
...@...
###.###
..#.#..
..#.#..
0 0

Output for the Sample Input

45
59
6
13

Solution in Haskell (judge verdict: Time Limit Exceeded)

main = interact $ unlines . map (show . red_black) . read' . lines
 where
  read' ("0 0":_) = []
  read' (wh:xs) = ((w,h), zip [0.. ] (concat tss)) : read' xs'
   where
    [w,h] = map read (words wh)
    (tss,xs') = splitAt h xs

red_black ((w,h),ts) = length (filter is_scanned (until_fix scan ts))
 where
  is_scanned (_,c) = c == '@'
  is_nonreach (_,c) = c == '.'
  replace (i,'.') = (i,'@')
  scan ts' = map paint ts'
   where
    paint t = if is_nonreach t && is_adjacent t then replace t else t
    is_adjacent (i,_) = (not.null) (filter is_scanned neighbors)
     where
      neighbors = filter (\(i',_) -> elem i' [i-1,i+1,i-w,i+w] && is_inborder i') ts'
      is_inborder i' = div i' w == div i w || abs (i' - i) == w

until_fix f x = if f x == x then x else until_fix f (f x)