open System
type Sol() =
    member this.Solve() = 
        let (Px,Py) = Console.ReadLine().Split(' ') |> Array.map double |> fun xs -> (xs.[0],xs.[1])
        let (Qx,Qy) = Console.ReadLine().Split(' ') |> Array.map double |> fun xs -> (xs.[0],xs.[1])
        let ans = abs(Px - Qx) + abs(Py - Qy) |> fun x -> x/2.0
        Console.WriteLine(ans)

let mySol = new Sol()
mySol.Solve()