Scanf.scanf "%d" (fun n ->
    let rec loop i stack =
        if i = n then List.hd stack else
            let stack =
                match Scanf.scanf " %s" (fun s -> s) with
                | "+" -> (
                    match stack with
                    | x :: y :: tl -> (x + y) :: tl
                    | _ -> stack
                )
                | "-" -> (
                    match stack with
                    | x :: y :: tl -> (y - x) :: tl
                    | _ -> stack
                )
                | a -> int_of_string a :: stack
            in
            loop (i + 1) stack
    in
    loop 0 [] |> Printf.printf "%d\n"
)