import kotlin.math.sqrt

fun main(args: Array<String>) {
    val n = readInt()
    val x = mutableListOf<Int>()
    val y = mutableListOf<Int>()
    repeat(n) {
        val (j, k) = readIntList()
        x.add(j)
        y.add(k)
    }

    val perm = permute((0 until n).toList())

    var ans = 0.0

    perm.forEach { order ->
        order.forEachIndexed { index, i ->
            if (index < n - 1) {
                val next = order[index + 1]
                ans += sqrt(((x[i] - x[next]) * (x[i] - x[next]) + (y[i] - y[next]) * (y[i] - y[next])).toDouble())
            }
        }
    }

    println(ans / perm.size)
}

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

fun <T> permute(list: List<T>): List<List<T>> {
    if (list.size == 1) return listOf(list)
    val perms = mutableListOf<List<T>>()
    val sub = list[0]
    for (perm in permute(list.drop(1)))
        for (i in 0..perm.size) {
            val newPerm = perm.toMutableList()
            newPerm.add(i, sub)
            perms.add(newPerm)
        }
    return perms
}