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 ans b, m, n
  r = 1
  while true
    r_new = r
    m.times do |m0|
      n.times do |n0|
        if b[n0][m0] == '@' then

          if (n0 - 1 >= 0) and (b[n0-1][m0] == '.') then
            b[n0-1][m0] = '@'
            r_new += 1
          end

          if (n0 + 1 < n) and (b[n0+1][m0] == '.') then
            b[n0+1][m0] = '@'
            r_new += 1
          end

          if (m0 - 1 >= 0) and (b[n0][m0-1] == '.') then
            b[n0][m0-1] = '@'
            r_new += 1
          end

          if (m0 + 1 < m) and (b[n0][m0+1] == '.') then
            b[n0][m0+1] = '@'
            r_new += 1
          end
        end
      end
    end
    if r_new == r then
      break
    else
      r = r_new
    end      
  end
  return r
end

while true do

  mn = $stdin.gets.chop.split(/ /).map { |x| x.to_i }
  m = mn[0]
  n = mn[1]

  if m == 0 and n == 0 then
    break
  else
    b = []
    n.times do
      b.push $stdin.gets.chomp
    end

    r = ans b, m, n
    printf("%d\n", r)
  end
  
end