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)

loop do
  w, h = gets.split.map(&:to_i)
  break if h == 0

  field = h.times.map do
    gets.chomp.split(//)
  end


  i, j = -> () {
    field.each.with_index do |f, i|
      f.each.with_index do |c, j|
        return [i,j] if c == '@'
      end
    end
  }.()

  field[i][j] = '.'

  walk = -> (x, y) {
    return 0 if x < 0 || y < 0 || x >= h || y >= w
    return 0 if field[x][y] != '.'
    field[x][y] = 'x'
    return 1 + 
      walk.(x+1, y) +
      walk.(x-1, y) +
      walk.(x, y-1) +
      walk.(x, y+1)
  }

  cnt = walk.(i, j)

  puts cnt
end