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 collections import deque

dx,dy = [1,0,-1,0],[0,1,0,-1]

while True:
	w,h = map(int,input().split())
	if w == 0 and h == 0:
		break
	field = [input() for i in range(h)]
	for i in range(h):
		if '@' in field[i]:
			sx,sy = i,field[i].find('@')
	visited = [[False for i in range(w)] for j in range(h)]
	que = deque()
	que.append((sx,sy))
	visited[sx][sy] = True
	while que:
		p = que.popleft()
		for i in range(4):
			nx,ny = p[0] + dx[i],p[1] + dy[i]
			if 0 <= nx < h and 0 <= ny < w and field[nx][ny] != '#' and not visited[nx][ny]:
				visited[nx][ny] = True
				que.append((nx,ny))
	cnt = 0
	for i in range(h):
		for j in range(w):
			cnt += visited[i][j]
	print(cnt)