fun main(args: Array<String>) {
	val n = (readLine() ?: return).toInt()
	val xy = Array(n) { (readLine() ?: return).split(" ").map { it.toInt() }.toIntArray() }
	var sum = 0.0
	var count = 0
	for (i in 0 until n) for (j in i + 1 until n) {
		sum += Math.sqrt((xy[i][0] - xy[j][0]) * (xy[i][0] - xy[j][0]) + (xy[i][1] - xy[j][1]) * (xy[i][1] - xy[j][1]).toDouble())
		count++
	}
	println(sum * (n - 1) / count)
}