fun main(args: Array<String>) {
    abc145c()
}

fun abc145c() {
    val n = readLine()!!.toInt()
    val xyList = (1..n).map { readLine()!!.split(' ').map { it.toDouble() } }
        .map { Pair(it[0], it[1]) }.toMutableList()

    var sum = 0.0
    for (i in 0..xyList.lastIndex) {
        val copy = xyList.toMutableList()
        val point = copy[i]
        copy.removeAt(i)
        copy.map {
            sum += Math.sqrt(
                Math.pow(it.first - point.first, 2.0) + Math.pow(
                    it.second - point.second,
                    2.0
                )
            )
        }
    }

    val answer = sum / n

    println(answer)
}
