#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
//#define int long long
using namespace std;

int H, W;
char grid[3000][3000];
int dp[3000][3000];

int min3(int a, int b, int c)
{
	return min(a, min(b, c));
}

int getLargestSquare(int H, int W)
{
	bool riku_exist = false;
	rep(i,H)
	{
		rep(j,W)
		{
			if (grid[i][j] == '.')
			{
				dp[i][j] = 0;
			}
			else if (grid[i][j] == '#')
			{
				dp[i][j] = 1;
				riku_exist = true;
			}
		}
	}

	if (!riku_exist)
	{
		return 0;
	}

	if (H == 1 || W == 1)
	{
		if (riku_exist)
		{
			return 1;
		}
		else
		{
			return 0;
		}
	}

	int maxWidth = 0;
	for (int i=1; i<H; i++)
	{
		for (int j=1; j<W; j++)
		{
			if (grid[i][j] == '.')
			{
				dp[i][j] = 0;
			}
			else
			{
				dp[i][j] = min3(dp[i - 1][j - 1], dp[i - 1][j], dp[i][j - 1]) + 1;
				maxWidth = max(maxWidth, dp[i][j]);
			}
		}
	}

	return maxWidth;
}

signed main()
{
	//入力
	cin >> H >> W;
	rep(i,H)
	{
		cin >> grid[i];
	}

	int len = getLargestSquare(H, W);
	if (len % 2 == 0)
	{
		printf("%d\n", len / 2);
	}
	else
	{
		printf("%d\n", (len + 1) / 2);
	}
}