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 Python3 (judge verdict: Accepted)

def location(field, wanted):
    for i, row in enumerate(field):
        for j, cell in enumerate(row):
            if cell == wanted:
                return i, j
def count(field, wanted):
    result = 0
    for row in field:
        for cell in row:
            if cell == wanted:
                result += 1
    return result
def neighborhood(center, w, h):
    i, j = center
    return filter(lambda x: 0 <= x[0] and x[0] < h and 0 <= x[1] and x[1] < w,
                  [                (i - 1, j    ),
                   (i    , j - 1),                 (i    , j + 1),
                                   (i + 1, j    )])
printed_later = []
while True:
    #入力
    w, h = map(int, input().split())
    if(w == 0 and h == 0):
        break
    field = []  # strの配列すなわちcharの配列の配列 '.':黒 '#':赤 '@':スタート (',':到達可能)
    for i in range(h):
        field.append(list(input()))
    #出力　幅優先探索
    queue = [location(field,'@')]
    field[queue[0][0]][queue[0][1]] = ','
    while len(queue) > 0:
        for cell in neighborhood(queue.pop(0), w, h):
            if field[cell[0]][cell[1]] == '.':
                queue.append(cell)
                field[cell[0]][cell[1]] = ','
    printed_later.append(count(field, ','))
for line in printed_later:
    print(line)