import java.lang.Math

fun main(args: Array<String>)
{
    val n = readLine()!!.toInt()
    val p = mutableListOf<Pair<Double, Double>>()
    for (i in 0 until n) {
        val (x, y) = readLine()!!.split(" ").map{it.toDouble()}
        p.add(Pair<Double, Double>(x, y))
    }

    val dist = mutableListOf<Double>()
    for (i in 0 until n-1) {
        for (k in i+1 until n) {
            val dx = p[i].first - p[k].first
            val dy = p[i].second - p[k].second
            val d = Math.sqrt(dx * dx + dy * dy)
            dist.add(d)
        }
    }

    var sum = 0.0
    for (i in 0..dist.lastIndex) sum += dist[i]

    println(sum * 2.0 / n.toDouble())
}
