let split_on_char sep s =
  let r = ref [] in
  let j = ref (String.length s) in
  for i = String.length s - 1 downto 0 do
    if String.unsafe_get s i = sep then begin
      r := String.sub s (i + 1) (!j - i - 1) :: !r;
      j := i
    end
  done;
  String.sub s 0 !j :: !r;;

let rec saiteki_henkan max_henkan gohan modosu =
  if (max_henkan > gohan) then
    saiteki_henkan (max_henkan - 1) (gohan + modosu) modosu
  else
    min max_henkan gohan;;


let cooking a b n m =
  (min a b) + (if (a > b)
               then
                 saiteki_henkan ((a - b) / n) ((a - b) mod n) n
               else
                 saiteki_henkan ((b - a) / m) ((b - a) mod m) m);;

let () =
  let raw = read_line () |> split_on_char ' ' |> List.map int_of_string in
  let a = List.nth raw 0 in
  let b = List.nth raw 1 in
  let n = List.nth raw 2 in
  let m = List.nth raw 3 in
  cooking a b n m |> print_int; print_newline ();;