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

let get() =
    stdin.ReadToEnd().Trim().Split [|' '; '\n'|]
    |> Array.map float

let solve input =
    match input with
    | [|Px; Py; Qx; Qy|] ->
        let diff k = abs << (-) k
        diff Px Qx + diff Py Qy |> (/) <| 2.0
    | _ -> failwith "Err"

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