open System;
 
let dprintfn fmt = Printf.kprintf Diagnostics.Debug.WriteLine fmt
 
[<EntryPoint>]
let main argv = 
    let px, py = Console.ReadLine().Trim().Split(' ') |> Array.map int |> fun a -> a.[0], a.[1]
    let qx, qy = Console.ReadLine().Trim().Split(' ') |> Array.map int |> fun a -> a.[0], a.[1]
    Console.WriteLine(float(abs(px - qx) + abs(py - qy)) / 2.0)
    0