var input = require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n");
var obj = {
  "list" : input,
  "index" : 0,
  "max" : input.length,
  "next" : function(){
    if(!this.hasNext()){throw "NoSuchElementException:次に要素は無いよ";}
    var returnObj = this.list[this.index];
    this.index++;
    return returnObj;
  },
  "hasNext" : function(){return (this.index < this.max);}
}
Main();
function next(){return obj.next();}
function hasNext(){return obj.hasNext();}
function myout(t){console.log(t);}//standard output
function myerr(t){console.error(t);}//standard error
//[no]要素の扱い。数値型
//不明値、異常時:引数そのまま返す　1:数値へ変換
//2:半角SPで分割 4:半角SPで分割し、数値配列へ
//6:1文字で分割  7:1文字で分割し、数値配列へ
//8:半角SPで結合 9:改行で結合 0:文字なしで結合
function myconv(i,no){try{switch(no){case 1:return parseInt(i);case 2:return i.split(" ");case 4:return i.split(" ").map((a)=>Number(a));case 6:return i.split("");case 7:return i.split("").map((a)=>Number(a));case 8:return i.join(" ");case 9:return i.join("\n");case 0:return i.join("");default:return i;}}catch(e){return i;}}
function Main(){
  var one = myconv(next(),4);
  var H = one[0];
  var W = one[1];
  var queue = new Array(1000000);
  var qIndex = 0;
  var masu = new Array(H);
  for(var i = 0; i < H; i++){
    masu[i] = myconv(next(),6);
    for(var j = 0; j < W; j++){
      if(masu[i][j] == "#"){
        queue[qIndex] = [i,j,0];
        qIndex++;
      }
    }
  }
  var output = 0;
  var index = 0;
  while(queue[index] != null){
    //var tmpQ = queue.shift();
    var tmpQ = queue[index];
    index++;
    var y = tmpQ[0];
    var x = tmpQ[1];
    var count = tmpQ[2];
    output = Math.max(output, count);
    masu[y][x] = (masu[y][x] == "." || masu[y][x] == "#") ? count : masu[y][x];
    if(y != 0){
       if(masu[y - 1][x] == "."){
          queue[qIndex] = [y - 1,x,count + 1];
          qIndex++;
       }
    }
    if(x != 0){
       if(masu[y][x - 1] == "."){
         queue[qIndex] = [y,x - 1,count + 1];
          qIndex++;
       }
    }
    if(y != H - 1){
       if(masu[y + 1][x] == "."){
         queue[qIndex] = [y + 1,x,count + 1];
          qIndex++;
       }
    }
    if(x != W - 1){
       if(masu[y][x + 1] == "."){
         queue[qIndex] = [y,x + 1,count + 1];
          qIndex++;
       }
    }
  }
  myout(output);
}
