import java.util.*

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)
    val n = sc.nextInt()
    val xy = (0 until n).map { sc.next().toDouble() to sc.next().toDouble() }
    var str = ""
    for (i in 0 until n) {
        str += i.toString()
    }
    val p = permutation(str,"")
    //println(p)
    //println(xy)
    //println(p)
    
    var distsum = 0.0
    p.forEach {
        for (j in 1 until n) {
            val j1 = it[j].toString().toInt()
            val j2 = it[j-1].toString().toInt()
            val dx = Math.pow(xy[j1].first - xy[j2].first,2.0)
            val dy = Math.pow(xy[j1].second - xy[j2].second,2.0)
            distsum += Math.sqrt(dx + dy)
        }
    }
    println(distsum / p.size)
 }

fun permutation(q:String,ans:String) : MutableList<String>{
    // Base Case
    val ret :MutableList<String> = mutableListOf()
    if (q.length <= 1) {
        ret.add(ans + q)
    }
    // General Case
    else {
        for (i in 0 until q.length) {
            ret.addAll( permutation(q.substring(0, i) + q.substring(i + 1),ans + q[i]) )
        }
    }
    return ret
}