import java.util.*

fun main(args:Array<String>){
    val n = readLine()!!.toInt()
    val a = (1 .. n).map{readLine()!!.split(" ").map{it.toDouble()}}
    var b = Array<Boolean>(n){false}
    var li = ArrayList<Int>(0)
    var sum : Double = 0.0
    fun dfs(){
        var ch = true
        for(i in 0 until n){
            if(!b[i]){
                ch = false
                b[i] = true
                li.add(i)
                dfs()
                li.remove(i)
                b[i] = false
            }
        }
        if(ch){
            var s : Double = 0.0
            for(i in 1 until n){
                s += Math.sqrt((a[li[i]][0] - a[li[i - 1]][0]) * (a[li[i]][0] - a[li[i - 1]][0]) + (a[li[i]][1] - a[li[i - 1]][1]) * (a[li[i]][1] - a[li[i - 1]][1]))
            }
            sum += s
        }
    }
    dfs()
    println("%.10f".format(sum / (1 .. n).map{it.toDouble()}.reduce{a1, a2 -> a1 * a2}))
}