object Main extends App {
    val in = io.Source.stdin.getLines.map(_.split(" ").map(_.toInt))
    while (in.next.head != 0) {
        val x = in.next.foldLeft((0.0, 0.0, 0)){case ((m, mean, n), d) =>
            (m - ((d + n * mean) / (n + 1) - d) * (d - mean), mean + (d - mean) / (n + 1), n + 1)
        }
        println(math.sqrt(x._1 / x._3))
    }
}