import sys

# sys.setrecursionlimit(200005)
# sys.set_int_max_str_digits(1000005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = -1-(-1 << 63)
# md = 10**9+7
md = 998244353

class CumSum2D:
    def __init__(self, arr, w=None):
        if w:
            self.h = arr
            self.w = w
        else:
            self.h = len(arr)
            self.w = len(arr[0])
        self.t = [0]*(self.h+1)*(self.w+1)
        if w == None:
            w = self.w+1
            for i, row in enumerate(arr):
                self.t[(i+1)*w+1:(i+1)*w+w] = row
        self._finalized = False

    def add(self, i, j, c):
        w = self.w+1
        self.t[(i+1)*w+j+1] += c

    def finalize(self):
        w = self.w+1
        self._finalized = True
        for ij in range(w+1, (self.h+1)*w):
            i, j = divmod(ij, w)
            if j == 0: continue
            if j+1 < w: self.t[i*w+j+1] += self.t[i*w+j]
            self.t[i*w+j] += self.t[(i-1)*w+j]

    def sum(self, ij0, ij1=None):
        if not self._finalized:
            print("Finalize!!!")
            exit()
        w = self.w+1
        i, j = ij0
        x, y = self.h, self.w
        if ij1: x, y = ij1
        if i >= x or j >= y: return 0
        return self.t[i*w+j]+self.t[x*w+y]-self.t[i*w+y]-self.t[x*w+j]

def imos_add(t, w, i0, j0, i1, j1, c):
    t[i0*(w+1)+j0] += c
    t[i1*(w+1)+j1] += c
    t[i0*(w+1)+j1] -= c
    t[i1*(w+1)+j0] -= c

def imos_finalize(t, w):
    w += 1
    for ij in range(len(t)-w):
        i, j = divmod(ij, w)
        if j == w-1: continue
        t[i*w+j+1] += t[i*w+j]
        if i: t[i*w+j] += t[(i-1)*w+j]

def binary_search(l, r, minimize):
    if minimize: l -= 1
    else: r += 1
    while l+1 < r:
        m = (l+r)//2
        if ok(m) ^ minimize: l = m
        else: r = m
    if minimize: return r
    return l

def ok(m):
    m2 = m**2
    cur = [0]*(h+1)*(w+1)
    for i in range(h-m+1):
        for j in range(w-m+1):
            if cs.sum([i, j], [i+m, j+m]) == m2:
                imos_add(cur,w,i,j,i+m,j+m,1)
    imos_finalize(cur,w)
    for i in range(h):
        for j in range(w):
            if (cur[i*(w+1)+j] > 0) != (ss[i][j] > 0): return False
    return True

h, w = LI()
ss = [[(c == "#")*1 for c in SI()] for _ in range(h)]
cs = CumSum2D(ss)
cs.finalize()
tt = []
for s in ss: tt += s+[0]

print(binary_search(1, min(h, w), False))