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 Ruby (judge verdict: Accepted)

@dx = [-1, 0, 1, 0]
@dy = [0, 1, 0, -1]
@field = []

def is_inside(x, y)
  return x >= 0 && y >= 0 && x < @field[0].length && y < @field.length
end

def count(x, y)
  ret = 1
  @field[y][x] = '#'
  (0 ... @dx.length).each do |i|
    if is_inside(x+@dx[i], y+@dy[i]) and @field[y+@dy[i]][x+@dx[i]] == '.' then
      ret += count(x+@dx[i], y+@dy[i])
    end
  end
  return ret
end

until (n, m = gets.split.map(&:to_i)) == [0, 0]
  @field = []
  m.times {@field << gets.chomp}
  x = 0
  y = 0
  (0 ... m).each do |i|
    x = @field[i].index('@')
    if x != nil then
      y = i
      break
    end
  end
  puts count(x, y)
end