let read () = Scanf.scanf "%d %d " (fun x y -> x, y)

let () =
  let px, py = read () in
  let qx, qy = read () in
  let x = (float qx -. float px) /. 2. |> abs_float in
  let y = (float qy -. float py) /. 2. |> abs_float in
  Printf.printf "%.1f\n" (x +. y)