H, W = map(int, raw_input().split())
frontier = set((i, -1) for i in xrange(H)) | set((i, W) for i in xrange(H)) | set((-1, j) for j in xrange(W)) | set((H, j) for j in xrange(W))
visited = set()
for i in xrange(H):
    b = raw_input()
    for j in xrange(W):
        if b[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
HW = (H+2)*(W+2)-4
while len(visited) < HW:
    ans += 1
    new_frontier = set()
    print sorted(frontier)
    for r, c in frontier:
        for i in xrange(8):
            nr, nc = r+dr[i], c+dc[i]
            if nr < 0 or nr >= H or nc < 0 or nc >= W:
                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