h, w = map(int, input().split())
s = [input() for _ in range(h)]
v = [[0] * (w + 1) for _ in range(h + 1)]
for i in range(h):
	for j in range(w):
		if s[i][j] == '#': v[i + 1][j + 1] += 1
		v[i + 1][j + 1] += v[i + 1][j] + v[i][j + 1] - v[i][j]
def f(x):
	c = [[0] * (w + 2) for _ in range(h + 2)]
	for i in range(h - x + 1):
		for j in range(w - x + 1):
			if v[i + x][j + x] - v[i + x][j] - v[i][j + x] + v[i][j] == x * x:
				k, l = i + 1, j + 1
				c[k][l] += 1
				c[k + x][l] -= 1
				c[k][l + x] -= 1
				c[k + x][l + x] += 1
	for i in range(h + 1):
		for j in range(w):
			c[i][j + 1] += c[i][j]
	for i in range(h):
		for j in range(w + 1):
			c[i + 1][j] += c[i][j]
	for i in range(h):
		for j in range(w):
			if s[i][j] == '#' and c[i + 1][j + 1] == 0:
				return 0
	return 1
l = 1
r = min(h, w) + 1
while l + 1 < r:
	m = (l + r) // 2
	if f(m): l = m
	else: r = m
print(l)