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)

def move(x,y,a)
  return if(x < 0)
  return if(y < 0)
  return if(a.size <= y)
  return if(a[0].size <= x)
  return if(a[y][x] == "#")
  a[y][x] = "#"
  @ans += 1
  move(x + 1, y ,a)
  move(x - 1, y, a)
  move(x, y + 1, a)
  move(x, y - 1, a)
end
while (true)
  w,h = gets.split.map &:to_i
  break if(w == 0)
  d = []
  @ans = 0
  x = y = 0
  h.times do |i|
    d[i] = gets.chomp
    if(d[i].include?("@"))
      y = i
      x = d[i].index("@")
    end
  end
  move(x, y, d)
  p @ans
end