import kotlin.math.hypot

fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val xy = (1..n).map {
        readLine()!!.split(" ").let {
            Pair(it[0].toDouble(), it[1].toDouble())
        }
    }
    println(xy.map { (x1, y1) ->
        xy.map { (x2, y2) ->
            hypot(x1 - x2, y1 - y2)
        }.sum()
    }.sum() / n)
}
