module S = Set.Make (struct
  type t = int
  let compare = (-)
end)

let () =
  let w, h, n = Scanf.scanf "%d %d %d " (fun w h n -> w, h, n) in
  let ss = Array.init w (fun _ ->
    let rec init i s =
      if i = h then s
      else S.add i s |> init (i + 1) in
    init 0 S.empty) in
  let a = Array.make w S.empty in
  let rec read i ks =
    if i = n then List.length ks
    else begin
      let s, k = Scanf.scanf "%d %d " (fun s k -> s, k) in
      let s, k = s - 1, k - 1 in
      a.(s) <- S.add (k) a.(s);
      read (i + 1) (if List.mem k ks then ks else k :: ks)
    end in
  let ks = read 0 [] in
  let rec doit i acc =
    if i = w then acc
    else if not (S.is_empty a.(i)) then begin
      let t = S.diff ss.(i) a.(i) in
      ss.(i) <- S.empty;
      doit (i + 1) (acc + S.cardinal t)
    end else doit (i + 1) (acc + ks) in
  doit 0 0 |> Printf.printf "%d\n"