fun main(args: Array<String>) {
    val N = readInt()
    val list = (0 until N).map{readIntList()}.toList()
    var ans = 0.0
    var num = 0

    fun f(xy:List<Int>, sum:Double, mada:List<Int>){
        if (mada.size == 0){
            ans += sum
            num++
            return
        }
        for (n in mada){
            val dist = Math.sqrt((xy[0]-list[n][0]).pow(2) + (xy[1]-list[n][1]).pow(2))
            f(list[n], sum+dist, mada.minus(n))
        }
    }
    val mada = (0 until N).toList()
    for (n in mada)
        f(list[n], 0.0, mada.minus(n))

    println(ans/num)
}

fun readString() = readLine()!!
fun readInt() = readLine()!!.toInt()
fun readLong() = readLine()!!.toLong()
fun readIntList() = readLine()!!.split(" ").map(String::toInt)
fun readLongList() = readLine()!!.split(" ").map(String::toLong)

fun Int.pow(n:Int) = Math.pow(this.toDouble(), n.toDouble())