let () =
  let k, n = read_line () |> Str.split (Str.regexp_string " ") |>
             List.map int_of_string |>
             fun lst -> (List.nth lst 0, List.nth lst 1)
  and alst = read_line () |> Str.split (Str.regexp_string " ") |>
             List.map int_of_string
  in
  let ans = (k * n) - (List.fold_left ( + ) 0 alst) in
  (if ans < 0 then -1 else ans) |> print_int;
  print_newline ()