fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

fun findAns a b =
    if a = b then "0"
    else if a < b then "+" ^ (Int.toString (b - a))
    else "-" ^ (Int.toString (a - b))


val () =
    let
        val px = readInt ()
        val py = readInt ()
        val qx = readInt ()
        val qy = readInt ()
       
        val ans = Real.fromInt (abs (px - qx) + abs (py - qy)) / 2.0
    in
        print (Real.toString ans ^ "\n")
    end