H, W = map(int, input().split())
S = [input() for _ in range(H)]
A = [[0] * (W + 1) for _ in range(H + 1)]
for i in range(H):
	for j in range(W):
		if S[i][j] == '#':
			A[i + 1][j + 1] += 1
for i in range(H + 1):
	for j in range(W):
		A[i][j + 1] += A[i][j]
for i in range(H):
	for j in range(W + 1):
		A[i + 1][j] += A[i][j]
le = 1
ri = (min(H, W) + 1) // 2
while le + 1 < ri:
	mi = (le + ri) // 2
	k = 2 * mi + 1
	ok = False
	for i in range(H + 1 - k):
		for j in range(W + 1 - k):
			if A[i + k][j + k] - A[i][j + k] - A[i + k][j] + A[i][j] == k * k:
				ok = True
	if ok:
		le = mi
	else:
		ri = mi
print(ri)