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)

from queue import Queue

def calcReachableTileNumByBFSearch(room, start_pos):
    q = Queue()
    q.put(start_pos)

    count = 1
    while not q.empty():
        cur_pos = q.get()
        D0 = [1, 0, -1, 0]
        D1 = [0, -1, 0, 1]
        for d in range(4):
            nc = [0,0]
            nc[0] = cur_pos[0]+D0[d]
            nc[1] = cur_pos[1]+D1[d]
            if room[nc[0]][nc[1]] == '.':
                room[nc[0]][nc[1]] = '*'
                q.put([nc[0],nc[1]])
                count +=1
    return count


def solve(width, height):
    room = [['#' for i in range(width+2)] for j in range(height+2)]
    for i in range(height):
        row = input()
        for j in range(width):
            room[i+1][j+1] = row[j]
            if room[i+1][j+1] == '@':
                start = [i+1,j+1]

    return calcReachableTileNumByBFSearch(room, start)


def main():
    while True:
        width, height = map(int, input().split())
        if width == 0 and height == 0:
            break
        print(solve(width, height))


if __name__ == "__main__":
    main()