#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
// using namespace atcoder;
// using mint = modint1000000007;
// const int mod = 1000000007;
// using mint = modint998244353;
// const int mod = 998244353;
const int INF = 1e9;
// const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i, l, r) for (int i = (l); i < (r); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rrep2(i, l, r) for (int i = (r)-1; i >= (l); --i)
#define all(x) (x).begin(), (x).end()
#define allR(x) (x).rbegin(), (x).rend()
#define P pair<int, int>
template<typename A, typename B> inline bool chmax(A &a, const B &b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A &a, const B &b) { if (a > b) { a = b; return true; } return false; }
const int dx8[8] = { 1,0,-1,0,1,1,-1,-1 };
const int dy8[8] = { 0,-1,0,1,1,-1,1,-1 };
int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int h, w; cin >> h >> w;
	vector<string>s(h);
	rep(i, h)cin >> s[i];
	vector dp(h, vector<int>(w, INF));
	queue<P>q;
	rep(i, h)rep(j, w)if (s[i][j] != '#') {
		q.emplace(i, j);
		dp[i][j] = 0;
	}
	rep(i, h)rep(j, w)if (s[i][j] == '#') {
		if (i == 0 || i == h - 1 || j == 0 || j == j - 1) {
			q.emplace(i, j);
			dp[i][j] = 1;
		}
	}
	while (!q.empty()) {
		auto[x, y] = q.front();
		q.pop();
		rep(t, 8) {
			int nx = x + dx8[t];
			int ny = y + dy8[t];
			if (nx < 0 || nx >= h)continue;
			if (ny < 0 || ny >= w)continue;
			if (chmin(dp[nx][ny], dp[x][y] + 1))q.emplace(nx, ny);
		}
	}
	int ans = 0;
	rep(i, h)rep(j, w)chmax(ans, dp[i][j]);
	cout << ans << endl;
	return 0;
}