// 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 diffX = abs <| Px - Qx
    let diffY = abs <| Py - Qy
    (diffX + diffY) / 2.0

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