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


val () =
    let
        val n = Real.fromInt (readInt ())
                                 
        val ans = (15.0 + 5.0 * Math.sqrt 5.0) / 12.0 * n * n * n
    in
        print ((Real.fmt (StringCvt.FIX (SOME 20)) ans) ^ "\n")
    end