function is_dark(A){
  var darks = [];
  for(var i=0;i<A.length;i++){
    line_set = new Set(A[i])
    if(line_set.size==1 && A[i][0]=='#'){continue;}
    if(line_set.size==2 || line_set.size==1 && A[i][0]=='.'){return false;}
  }
  return true;
}

function main(input){
  const lines = input.split('\n');
  const H = lines[0].split(' ').map(x => parseInt(x))[0];
  const W = lines[0].split(' ').map(x => parseInt(x))[1];
  var A=[];
  for(var i=0;i<H;i++){
    var line = lines[1+i].split('');
    A.push(line);
  }

//  console.log([H,W,A]);

  var result = 0;

  // get black
  var blacks = [];
  for(var i=0;i<A.length;i++){
    for(var j=0;j<A[0].length;j++){
      if(A[i][j]=='#'){blacks.push([i,j])}
    }
  }

  while(true){

    var darken_blacks=[]
    for(var i=0;i<blacks.length;i++){
      var c_black = blacks[i];
      x = c_black[1];
      y = c_black[0];

      if((y>0 && A[y-1][x]=='#')&&(x>0 && A[y][x-1]=='#')&&(x<A[0].length-1 && A[y][x+1]=='#')&&(y<A.length-1 && A[y+1][x]=='#')){continue;}

      if(y>0                            && A[y-1][x]=='.'){
        darken_blacks.push([y-1, x]);
        A[y-1][x]='#';
      }
      if(x>0                            && A[y][x-1]=='.'){
        darken_blacks.push([y, x-1]);
        A[y][x-1]='#';
      }
      if(x<A[0].length-1 && A[y][x+1]=='.'){
        darken_blacks.push([y, x+1]);
        A[y][x+1]='#';
      }
      if(y<A.length-1       && A[y+1][x]=='.'){
        darken_blacks.push([y+1, x]);
        A[y+1][x]='#';
      }
    }
    blacks = darken_blacks;

    if(blacks.length==0){break;}
//    console.log('==');
//    console.log(blacks);
//    console.log(blacks.length);
//    console.log(A);
    result++;
  }

  console.log(result);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));
