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

let get() =
    stdin.ReadLine().Split [|' '|]
    |> Array.map float

let getValues() =
    match get() with
    | [|x; y|] -> x, y
    | _ -> failwith "Error"

let getInputs() =
    let Px, Py = getValues()
    let Qx, Qy = getValues()
    Px, Py, Qx, Qy

let solve(Px, Py, Qx, Qy) =
    let diff k = abs << (-) k
    let dx = diff Px Qx
    let dy = diff Py Qy
    let sum = (+) dx dy
    (/) sum 2.0

[<EntryPoint>]
let main _ =
    printfn "%.1f" << solve << getInputs <| ()
    0