let solve a b x y =
  if x *. b /. a <= y then x +. x *. b /. a
  else y +. y *. a /. b

let () =
  let a, b, x, y = read_line () |>
                   Str.split (Str.regexp_string " ") |>
                   List.map float_of_string |>
                   fun lst -> (List.nth lst 0, List.nth lst 1,
                               List.nth lst 2, List.nth lst 3)
  in
  solve a b x y |> Printf.printf "%.7f\n"