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 rec f arr x y d = 
  if 0 <= x && x < h && 0 <= y && y < w then
    if arr.(x).(y) >= d then begin
      arr.(x).(y) <- d;
      f arr (x + 1) y (d + 1);
      f arr (x - 1) y (d + 1);
      f arr x (y + 1) (d + 1);
      f arr x (y - 1) (d + 1);
    end

let () =
  for x = 0 to (h - 1) do
    for y = 0 to (w - 1) do
      if da.(x).(y) = 0 then f da x y 0
    done
  done;
  print_int @@ Array.fold_left max 0 @@ Array.map (Array.fold_left max 0) da