H, W = map(int, raw_input().split())
board = ['.'*(W+2)]
for i in xrange(H):
    board.append('.'+raw_input()+'.')
board.append('.'*(W+2))

frontier = set()
visited = set()
for i in xrange(H+2):
    for j in xrange(W+2):
        if board[i][j] == '.':
            frontier.add((i, j))


dr = [-1, -1, -1, 0, 0, 1, 1, 1]
dc = [-1, 0, 1, -1, 1, -1, 0, 1]
ans = 0
while len(visited) < (H+2)*(W+2):
    ans += 1
    new_frontier = set()
    for r, c in frontier:
        for i in xrange(8):
            nr, nc = r+dr[i], c+dc[i]
            if nr < 0 or nr >= H+2 or nc < 0 or nc >=W+2:
                continue
            if (nr, nc) in visited or (nr, nc) in frontier:
                continue
            new_frontier.add((nr, nc))
    visited |= frontier
    frontier = set(new_frontier)
print ans-1