#include <bits/stdc++.h>
using namespace std;
const int maxn = 1010;
int vis[maxn][maxn];
int main(){
	int n, m; cin >> n >> m;
	vector<string> g(n); for(auto & i : g) cin >> i;
	vector<pair<int,int> > q, nq;
	for(int i = 0; i < n; i++) for(int j = 0; j < m; j++) if(g[i][j] == '#') q.push_back({i, j}), vis[i][j] = 1;
	for(int i = 0; ; i++){
		for(auto e : q){
			int x, y; tie(x, y) = e;
			for(int di = -1; di <= 1; di++){
				for(int dj = -1; dj <= 1; dj++){
					if(abs(di) + abs(dj) != 1) continue;
					int nx = x + di, ny = y + dj;
					if(nx >= 0 && nx < n && ny >= 0 && ny < m && !vis[nx][ny]) nq.push_back({nx, ny}), vis[nx][ny] = 1;
				}
			}
		}
		if(nq.empty()){
			cout << i << endl;
			return 0;
		}
		q = nq;
		nq.clear();
	}
	return 0;
}
