let d x1 y1 x2 y2 = (x1 -. x2) ** 2. |> (+.) ((y1 -. y2) ** 2.) |> sqrt ;;
let () =
  let f = float_of_string in
  let x1, y1, x2, y2 = Scanf.scanf "%s %s %s %s\n" (fun x y z w -> f x, f y, f z, f w) in
  Printf.printf "%.10f\n" (d x1 y1 x2 y2) ;;
