let () =
  let k, n, f = Scanf.scanf "%d %d %d\n" (fun a b c -> a, b, c) in
  let lst = Array.to_list (Array.init f (fun _ -> Scanf.scanf "%d " (fun x -> x))) in
  let sum = List.fold_left (+) 0 lst in
  Printf.printf "%d\n" (if sum > k * n then -1 else k * n - sum)