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 = [[float('inf')] * W for i in range(H)]
    for w in range(W):
        if S[0][w] == '#':
            dist[0][w] = 1
        else:
            dist[0][w] = 0
    for h in range(1, H):
        Sh = S[h]
        disth = dist[h]
        disthm = dist[h - 1]
        for w, c in enumerate(Sh):
            if c == '#':
                if w:
                    disth[w] = min(disth[w - 1], disthm[w - 1], disthm[w]) + 1
                else:
                    disth[w] = 1
            else:
                disth[w] = 0
    maxd = 0
    for row in dist:
        maxd = max(maxd, max(row))
    return (maxd + 1) // 2

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