#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;
	while(~scanf("%d%d%*c", &H, &W)) {
		vector<unique_ptr<char[]>> S(H);
		rep(i, H) {
			S[i] = make_unique<char[]>(W + 2);
			fgets(S[i].get(), W + 2, stdin);
		}
		vector<vector<int>> dp(H + 1, vector<int>(W + 1));
		int m = 0;
		rep(i, H) rep(j, W) if(S[i][j] == '#') {
			int x = min({ dp[i][j + 1], dp[i + 1][j], dp[i][j] }) + 1;
			amax(m, x);
			dp[i + 1][j + 1] = x;
		}
		int ans = (m + 1) / 2;
		printf("%d\n", ans);
	}
	return 0;
}