#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)
{
	rep(i,H)
	{
		rep(j,W)
		{
			if (grid[i][j] == '.')
			{
				dp[i][j] = 0;
			}
			else if (grid[i][j] == '#')
			{
				dp[i][j] = 1;
			}
		}
	}

	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];
	}

	printf("%d\n", (getLargestSquare(H, W) + 1) / 2);
}