let f (s:System.String) = s.Split(' ') |> Seq.map int64
let m =
    stdin.ReadLine() |> f |> Seq.toList
    |> function
    | [k;n;f] -> k*n
    | _ -> failwith "error!"
let s = stdin.ReadLine() |> f |> Seq.sum
if s > m then -1L
else m-s
|> printfn "%d"