data class Point(val x: Double, val y: Double) {
    fun dist(o:Point): Double {
        return Math.sqrt((x-o.x)*(x-o.x) + (y-o.y)*(y-o.y))
    }
}
fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val P = mutableListOf<Point>()
    for (i in 0 until n) {
        val l: List<Int> = readLine()!!.split(" ").map {it.toInt()}
        P.add(Point(l[0].toDouble(), l[1].toDouble()))
    }
    var s = 0.0
    for (i in 0 until n) {
        for (j in i+1 until n) {
            s += P[i].dist(P[j])
        }
    }
    s = s * 2 / n
    println(s)
}
