H,W = map(int,raw_input().split())
assert 1 <= H <= 3000
assert 1 <= W <= 3000
S = [raw_input() for i in xrange(H)]
for i in xrange(H):
	assert len(S[i]) == W
	assert S[i].count("#")+S[i].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,H+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