let split s c =
  let len = String.length s in
  let rec iter pos to_rev =
    if pos = len then List.rev ("" :: to_rev) else
      match try
          Some ( String.index_from s pos c )
        with Not_found -> None
      with
        Some pos2 ->
        if pos2 = pos then iter (pos+1) ("" :: to_rev) else
          iter (pos2+1) ((String.sub s pos (pos2-pos)) :: to_rev)
      | None -> List.rev ( String.sub s pos (len-pos) :: to_rev )
  in
  iter 0 []

let beens k n agesum =
  let beensum = k * n in
  if beensum < agesum then -1 else
    beensum - agesum

let () =
  let beens1 = Scanf.sscanf (read_line ()) "%d %d" beens in
  let age_list = List.map int_of_string @@ split (read_line ()) ' ' in
  print_int @@ beens1 @@ List.fold_left (fun x y -> x + y) 0 age_list