#include <iostream>
#include <utility>
#include <unordered_set>
#include <string>
#include <functional>
using namespace std;

template <class T>
inline void hash_combine(std::size_t& seed, const T& v)
{
  hash<T> hasher;
  seed ^= hasher(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);
}

struct SimpleHash {
  size_t operator()(const std::pair<int, int>& p) const {
    size_t seed = 0;
    hash_combine(seed, p.first);
    hash_combine(seed, p.second);
    return seed;
  }
};

int main() {
  int H, W;
  cin >> H >> W;
  string s;
  std::unordered_set<std::pair<int, int>, SimpleHash> frontier;
  std::unordered_set<std::pair<int, int>, SimpleHash> new_frontier;
  std::unordered_set<std::pair<int, int>, SimpleHash> visited;
  for (int i = 0; i < H; i++) {
    cin >> s;
    for (int j = 0; j < W; j++) {
      if (s[j] == '.')
	frontier.emplace(i, j);
    }
  }
  for (int i = -1; i < H+1; i++) {
    frontier.emplace(i, -1);
    frontier.emplace(i, W);
  }
  for (int j = -1; j < W+1; j++) {
    frontier.emplace(-1, j);
    frontier.emplace(H, j);
  }

  int dr[8] = {-1, -1 , -1, 0, 0, 1, 1, 1};
  int dc[8] = {-1, 0, 1, -1, 1,  -1, 0, 1};
  int nr, nc;
  pair<int, int> nrnc;
  int ans = -1;
  while ((int)frontier.size() > 0) {
    ans += 1;
    new_frontier.clear();
    for (const auto& elem: frontier) {
      for (int i = 0; i < 8; i++) {
	nr = elem.first + dr[i];
	nc = elem.second+ dc[i];
	if (nr < 0 || nr >= H || nc < 0 || nc >= W)
	  continue;
	nrnc = pair<int, int>(nr, nc);
	if (visited.find(nrnc) != visited.end() || frontier.find(nrnc) != frontier.end())
	  continue;
	new_frontier.insert(nrnc);
      }
    }
    for (const auto& elem: frontier)
      visited.insert(elem);
    frontier.swap(new_frontier);
  }

  cout << ans << endl;
  return 0;
}