I,R=input,range
H,W=map(int,I().split())
X,Y=R(W),R(H)
S=[[s<'.'for s in I()]for i in Y]
C=[[0]*(W+1)for i in R(H+1)]
c=[0]*(W+1)
for i in Y:
	t=0
	for j in X:C[i+1][j+1],t=c[j+1]+t+S[i][j],t+S[i][j]
	c=C[i+1][:]
def r(k):
	D=[[0]*(W+1)for i in R(H+1)]
	for i in R(H-k+1):
		for j in R(W-k+1):
			if C[i+k][j+k]-C[i+k][j]-C[i][j+k]+C[i][j]==k*k:D[i+k][j+k],D[i+k][j],D[i][j+k],D[i][j]=D[i+k][j+k]+1,D[i+k][j]-1,D[i][j+k]-1,D[i][j]+1
	c=[0]*W
	for i in Y:
		t=0
		for j in X:D[i][j],t=c[j]+t+D[i][j],t+D[i][j]
		if any(S[i][j]>D[i][j]for j in X):return 1
		c=D[i][:]
	return 0
A=[1,min(H,W)+1]
while A[0]+1<A[1]:
	k=(A[0]+A[1])//2
	A[r(k)]=k
print(A[0])