def solve():
	H,W = map(int,raw_input().split())
#	assert 1 <= H <= 3000
#	assert 1 <= W <= 3000
	S = [raw_input() for i in xrange(H)]
#	for s in S:
#		assert len(s) == W
#		assert s.count("#") + s.count(".") == W
	HD = [[W]*(W+2) for i in xrange(H+2)]
	VD = [[H]*(W+2) for i in xrange(H+2)]
	for i in xrange(H+2):
		for j in xrange(W+2):
			if not (0 < i <= H and 0 < j <= W) or S[i-1][j-1] == ".":
				HD[i][j] = VD[i][j] = 0
	for i in xrange(1,H+1):
		for j in xrange(1,W+1):
			HD[i][j] = 1 + min(HD[i][j]-1,HD[i-1][j-1],HD[i][j-1],HD[i+1][j-1])
			VD[i][j] = 1 + min(VD[i][j]-1,VD[i-1][j-1],VD[i-1][j],VD[i-1][j+1])
	for i in xrange(H,0,-1):
		for j in xrange(W,0,-1):
			HD[i][j] = 1 + min(HD[i][j]-1,HD[i+1][j+1],HD[i][j+1],HD[i-1][j+1])
			VD[i][j] = 1 + min(VD[i][j]-1,VD[i+1][j+1],VD[i+1][j],VD[i+1][j-1])
	ans = 0
	for i in xrange(1,H+1):
		for j in xrange(1,W+1):
			ans = max(ans,min(HD[i][j],VD[i][j]))
	print ans
solve()