open System

let No410 () =
    let P = Console.ReadLine().Split(' ')
    let Q = Console.ReadLine().Split(' ')
    let s = Math.Abs(((Double.Parse P.[0]) - (Double.Parse Q.[0])))
    let s = s + Math.Abs((Double.Parse P.[1] - (Double.Parse Q.[1])))
    let s = s / 2.0
    printfn "%f" s
    ()

No410 ()