// 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