H, W = map(int, input().split())
grid = [input().strip() for _ in range(H)]

prefix = [[0] * (W + 1) for _ in range(H + 1)]
for i in range(1, H + 1):
    row_sum = 0
    for j in range(1, W + 1):
        row_sum += 1 if grid[i-1][j-1] == '#' else 0
        prefix[i][j] = prefix[i-1][j] + row_sum

def is_possible(k):
    if k == 0:
        return False
    diff = [[0] * (W + 2) for _ in range(H + 2)]
    max_a = H - k + 1
    max_b = W - k + 1
    for a in range(1, max_a + 1):
        a_end = a + k - 1
        for b in range(1, max_b + 1):
            b_end = b + k - 1
            total = prefix[a_end][b_end] - prefix[a-1][b_end] - prefix[a_end][b-1] + prefix[a-1][b-1]
            if total == k * k:
                diff[a][b] += 1
                diff[a][b_end + 1] -= 1
                diff[a_end + 1][b] -= 1
                diff[a_end + 1][b_end + 1] += 1

    # Compute row-wise prefix sums
    for i in range(1, H + 1):
        for j in range(1, W + 1):
            diff[i][j] += diff[i][j-1]
    # Compute column-wise prefix sums
    for j in range(1, W + 1):
        for i in range(1, H + 1):
            diff[i][j] += diff[i-1][j]

    # Check all black cells
    for i in range(H):
        for j in range(W):
            if grid[i][j] == '#' and diff[i+1][j+1] == 0:
                return False
    return True

low = 1
high = min(H, W)
ans = 0
while low <= high:
    mid = (low + high) // 2
    if is_possible(mid):
        ans = mid
        low = mid + 1
    else:
        high = mid - 1
print(ans)