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