def aa(m):
    global h,w
    for i in range(m,h+2-m):
        for j in range(m,w+2-m):
            if x[i+m-1][j+m-1]-x[i-m][j+m-1]-x[i+m-1][j-m]+x[i-m][j-m]==0:
                return 1
    return 0


h,w=map(int,input().split())
x=[[0]*(w+1) for i in range(h+1)]
for i in range(h):
    s=input()
    for j in range(w):
        if s[j]==".":
            x[i+1][j+1]=1
        x[i+1][j+1]+=x[i+1][j]
for i in range(1,h+1):
    for j in range(w+1):
        x[i][j]+=x[i-1][j]
q,e=1,2000
while e-q>1:
    m=(q+e)//2
    if aa(m):
        q=m
    else:
        e=m
print(q)