let id x = x
let split_string ?(pattern="") = Str.split @@ Str.regexp pattern

let (h, w) = Scanf.scanf "%d %d\n" @@ fun h w -> (h, w)
let a = Array.init h (fun _ -> Array.of_list (split_string (Scanf.scanf "%s\n" id)))

let da = Array.init h @@ fun x -> Array.init w @@ fun y -> if a.(x).(y) = "#" then 0 else max_int

let que = Queue.create ()

let add x y d = 
  List.iter (fun (x, y) ->
    if 0 <= x && x < h && 0 <= y && y < w && da.(x).(y) > d then Queue.add (x, y, d) que
  ) [(x + 1, y); (x - 1, y); (x, y + 1); (x, y - 1)]

let () =
  for x = 0 to (h - 1) do
    for y = 0 to (w - 1) do
      if da.(x).(y) = 0 then 
        add x y 1          
    done
  done;
  while not (Queue.is_empty que) do
    let (x, y, d) = Queue.pop que in
    if da.(x).(y) > d then begin
      da.(x).(y) <- d;
      add x y (d + 1)
    end
  done;
  print_int @@ Array.fold_left max 0 @@ Array.map (Array.fold_left max 0) da