// yukicoder My Practice
// author: Leonardone @ NEETSDKASU

let get() =
    stdin.ReadToEnd().Trim().Split [|' '; '\n'|]
    |> Array.map float
    |> fun xyxy ->
        match xyxy with
        | [|_; _; _; _|] -> xyxy
        | _ -> failwith "Error"

let solve [|Px; Py; Qx; Qy|] =
    let diff k = abs << (-) k
    diff Px Qx + diff Py Qy |> (/) <| 2.0

[<EntryPoint>]
let main _ =
    printfn "%A" << solve << get <| ()
    0