import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.complex, std.container, std.math, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int H, W;
string[] A;

void main() {
  try {
    for (; ; ) {
      H = readInt();
      W = readInt();
      A = new string[H];
      foreach (x; 0 .. H) {
        A[x] = readToken();
      }
      
      auto q = new int[2 * H * W];
      int qHead, qTail;
      auto dist = new int[][](H, W);
      foreach (x; 0 .. H) foreach (y; 0 .. W) {
        if (A[x][y] == '#') {
          dist[x][y] = 0;
          q[qTail++] = x;
          q[qTail++] = y;
        } else {
          dist[x][y] = H * W;
        }
      }
      for (; qHead != qTail; ) {
        const x = q[qHead++];
        const y = q[qHead++];
        void update(int xx, int yy) {
          if (chmin(dist[xx][yy], dist[x][y] + 1)) {
            q[qTail++] = xx;
            q[qTail++] = yy;
          }
        }
        if (x + 1 < H) {
          update(x + 1, y);
        }
        if (y + 1 < W) {
          update(x, y + 1);
        }
        if (x - 1 >= 0) {
          update(x - 1, y);
        }
        if (y - 1 >= 0) {
          update(x, y - 1);
        }
      }
      
      int ans;
      foreach (x; 0 .. H) foreach (y; 0 .. W) {
        chmax(ans, dist[x][y]);
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}
