import kotlin.math.hypot

fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val city = mutableListOf<Pair<Double, Double>>()
    for (i in 0..(n-1)) {
        val aa = readLine()!!.split(" ").map { it.toDouble() }.let {
            Pair(it[0], it[1])
        }
        city.add(aa)
    }

    var result = 0.0

    for (i in 0..(n-2)) {
        for (j in i..(n-1)) {
            result += hypot(city[i].first - city[j].first, city[i].second - city[j].second)
        }
    }
    println( 2.0 / n.toDouble() * result)
}