import java.lang.Math.pow
import java.lang.Math.sqrt

fun main(args: Array<String>) {
    var n = readLine()!!.toInt()
    var x = mutableListOf<Double>()
    var y = mutableListOf<Double>()
    for (i in 0 until n){
        var tmp = readLine()!!.split(" ").map { it.toDouble()}
        x.add(tmp[0])
        y.add(tmp[1])
    }
    var sum = 0.0
    for (i in 0 until n-1){
        for (j in i+1 until n){
            sum += sqrt(pow(x[i] - x[j], 2.0) + pow(y[i] - y[j] , 2.0))
        }
    }
    println(sum * 2 / n)
}