#include <cstdio>
#include <algorithm>
#define maxn 2000

int n, m;
char a[maxn + 5][maxn + 5];
int sum[maxn + 5][maxn + 5], tmp[maxn + 5][maxn + 5];

int main()
{
	scanf("%d%d", &n, &m);
	for (int i = 1; i <= n; ++i)
		scanf("%s", a[i] + 1);
	for (int i = 1; i <= n; ++i)
		for (int j = 1; j <= m; ++j)
			sum[i][j] = sum[i][j - 1] + sum[i - 1][j] - sum[i - 1][j - 1] + (a[i][j] == '#');
	int L = 1, R = std::min(n, m);
	for (; L < R; )
	{
		int mid = (L + R + 1) >> 1;
		for (int i = 1; i + mid - 1 <= n; ++i)
			for (int j = 1; j + mid - 1 <= m; ++j)
			{
				int _i = i + mid - 1, _j = j + mid - 1;
				int cnt = sum[_i][_j] - sum[_i][j - 1] - sum[i - 1][_j] + sum[i - 1][j - 1];
				if (cnt == mid * mid) ++tmp[i][j], --tmp[i][_j + 1], --tmp[_i + 1][j], ++tmp[_i + 1][_j + 1];
			}
		bool res = true;
		for (int i = 1; i <= n && res; ++i)
			for (int j = 1; j <= m && res; ++j)
			{
				tmp[i][j] = tmp[i][j - 1] + tmp[i - 1][j] - tmp[i - 1][j - 1] + tmp[i][j];
				if ((tmp[i][j] > 0 && a[i][j] == '.') || (tmp[i][j] == 0 && a[i][j] == '#')) res = false;
			}
		if (res)
			L = mid;
		else
			R = mid - 1;
	}
	printf("%d", L);
	return 0;
}