def read_data():
    H, W = map(int, input().split())
    S = [input() for i in range(H)]
    return H, W, S

def solve(H, W, S):
    dist = [[-1] * W for i in range(H)]
    pool = []
    for h in range(H):
        Sh = S[h]
        disth = dist[h]
        for w in range(W):
            if Sh[w] == '.':
                disth[w] = 0
                pool.append((h, w))
    if not pool:
        return (min(H, W) + 1) // 2
    for i in range(W):
        pool.append((-1, i))
        pool.append((H, i))
    for i in range(H):
        pool.append((i, -1))
        pool.append((i, W))
    d = 0
    while pool:
        d += 1
        newpool = []
        for h, w in pool:
            for dh, dw in [(1,1), (1,0), (1,-1), (0,1),(0,-1),(-1,1),(-1,0),(-1,-1)]:
                nh = dh + h
                nw = dw + w
                if nh < 0 or nh >= H or nw < 0 or nw >= W:
                    continue
                if S[nh][nw] == '.' or dist[nh][nw] != -1:
                    continue
                dist[nh][nw] = d
                newpool.append((nh, nw))
        pool = newpool
    return d - 1

H, W, S = read_data()
print(solve(H, W, S))