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)

class dfs:
    def __init__(self,W,H,m):
        self.W=W
        self.H=H
        self.m=m
        self.ans=0
        self.visited=[[False]*W for _ in range(H)]
    def trav(self,x,y):
        #print(x,y)
        self.visited[x][y]=True
        self.ans+=1
        offset=[(-1,0),(0,-1),(1,0),(0,1)]
        for dx,dy in offset:
            x2=x+dx
            y2=y+dy
            if 0<=x2<self.H and 0<=y2<self.W and self.m[x2][y2]=='.' and self.visited[x2][y2]==False:
                self.trav(x2,y2)
    

while True:
    W,H=map(int,input().split())
    if not (W and H): break
    m=[]
    for i in range(H):
        L=list(input())
        if '@' in L:
            (sx,sy)=(i,L.index('@'))
        m.append(L)
    D=dfs(W,H,m)
    D.trav(sx,sy)
    print(D.ans)