#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int h, w; cin >> h >> w;
	vector<string>s(h);
	rep(i, h)cin >> s[i];
	vector sc(h + 1, vector<int>(w + 1));
	rep(i, h)rep(j, w)if ('#' == s[i][j])sc[i + 1][j + 1]++;
	rep(i, h + 1)rep(j, w) sc[i][j + 1] += sc[i][j];
	rep(i, w + 1)rep(j, h) sc[j + 1][i] += sc[j][i];
	// 左上(x0,y0)右下(x1,y1)
	auto calc = [&](int x0, int y0, int x1, int y1)->long long {
		return  sc[x1 + 1][y1 + 1] - sc[x1 + 1][y0] - sc[x0][y1 + 1] + sc[x0][y0];
	};
	long long ng = max(h, w) + 1;
	long long ok = 1;
	while (1 < abs(ok - ng)) {
		long long mid = (ng + ok) / 2;
		//cout << mid << endl;
		vector imos(h + 1, vector<int>(w + 1));
		auto add = [&](int x0, int y0, int x1, int y1, int val)->void {
			imos[x0][y0] += val;
			imos[x0][y1] -= val;
			imos[x1][y0] -= val;
			imos[x1][y1] += val;
		};
		auto check = [&]() {
			rep(i, h - mid + 1)rep(j, w - mid + 1) {
				int cnt = calc(i, j, i + mid - 1, j + mid - 1);
				//cout << cnt << endl;
				if (cnt != mid * mid)continue;
				add(i, j, i + mid, j + mid, 1);
			}
			rep(i, h + 1) rep(j, w) imos[i][j + 1] += imos[i][j];
			rep(i, w + 1) rep(j, h) imos[j + 1][i] += imos[j][i];
			bool chk = true;
			rep(i, h)rep(j, w)if ('#' == s[i][j] && 0 == imos[i][j])chk = false;
			/*rep(i, h) {
				rep(j, w) {
					cout << imos[i][j];
				}
				cout << endl;
			}*/
			return chk;
		};
		if (check()) ok = mid;
		else ng = mid;
	}
	cout << ok << endl;
	return 0;
}