#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", &H, &W)) {
		H += 2, W += 2;
		vector<string> S(H);
		rep(i, H - 2) {
			cin >> S[i + 1];
			S[i + 1] = '.' + S[i + 1] + '.';
		}
		S[0] = S[H - 1] = string(W, '.');
		vector<vector<bool>> vis(H, vector<bool>(W));
		vector<pii> q, nq;
		rep(i, H) rep(j, W) if(S[i][j] == '.') {
			nq.emplace_back(i, j);
			vis[i][j] = true;
		}
		int dist = 0, ans = 0;
		while(!nq.empty()) {
			q.swap(nq);
			while(!q.empty()) {
				int i, j; tie(i, j) = q.back(); q.pop_back();
				amax(ans, dist);
				rer(dy, -1, 1) rer(dx, -1, 1) if(dy || dx) {
					int yy = i + dy, xx = j + dx;
					if(yy < 0 || yy >= H || xx < 0 || xx >= W) continue;
					if(!vis[yy][xx]) {
						vis[yy][xx] = true;
						nq.emplace_back(yy, xx);
					}
				}
			}
			++ dist;
		}
		printf("%d\n", ans);
	}
	return 0;
}