import java.util.*

fun main(args: Array<String>) {
    val scanner = Scanner(System.`in`)
    val N = scanner.nextInt()

    val points = mutableListOf<Point>()
    for (i in 1..N) {
        points.add(Point(scanner.nextInt(), scanner.nextInt()))
    }

    val distances: List<Double> = listChildDistances(null, 0.0, points)
    println(distances.average())
}

private fun listChildDistances(prevPoint: Point?, distanceToHere: Double, points: List<Point>): List<Double> {
    val distances = mutableListOf<Double>()
    for (point in points) {
        val distance: Double = if (prevPoint == null) {
            0.0
        } else {
            distance(prevPoint, point)
        }
        val nextPoints = points.filter { it != point }
        if (nextPoints.isEmpty()) {
            distances.add(distanceToHere + distance)
        } else {
            distances.addAll(listChildDistances(point, distanceToHere + distance, nextPoints))
        }
    }
    return distances
}

private data class Point(val x: Int, val y: Int)

private fun distance(p1: Point, p2: Point): Double = Math.sqrt((pow2(p1.x - p2.x) + pow2(p1.y - p2.y)).toDouble())

private fun pow2(i: Int): Int = i * i