#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if(x < y) x = y; }

int main() {
	int H; int W;
	scanf("%d%d", &H, &W);
	int dp[3001] = {};
	char S[3001];
	int m = 0;
	rep(i, H) {
		scanf("%s", S);
		int prevx = 0, prevt = 0;
		rep(j, W) {
			int x;
			int t = dp[j];
			if(S[j] == '#') {
				x = min({ prevt, t, prevx }) + 1;
				amax(m, x);
			} else {
				x = 0;
			}
			dp[j] = prevx = x;
			prevt = t;
		}
	}
	int ans = (m + 1) / 2;
	printf("%d\n", ans);
	return 0;
}