open Num

let solve vl vr d w =
  let t = d // (vl +/ vr) in
  t */ w

let () =
  let vl, vr = read_line () |>
               Str.split (Str.regexp_string " ") |>
               List.map num_of_string |>
               fun lst -> (List.nth lst 0, List.nth lst 1)
  and d = read_line () |> num_of_string
  and w = read_line () |> num_of_string
  in
  solve vl vr d w |> float_of_num |> (Printf.printf "%.7f")