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


val () =
    let
        val xa = readInt ()
        val ya = readInt ()
        val xb = readInt ()
        val yb = readInt ()

        val ans = (Real.fromInt (yb * (xb + xa) - (yb - ya) * xb)) / (Real.fromInt (xb + xa))
    in
        print ((Real.fmt (StringCvt.FIX (SOME 6)) ans) ^ "\n")
    end