package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
	"strings"
)

type axis struct {
	x int
	y int
}

func main() {
	io, flush := NewIO()
	defer flush()

	n := io.ScanInt()
	towns := make([]*axis, n)
	for i := 0; i < n; i++ {
		towns[i] = &axis{x: io.ScanInt(), y: io.ScanInt()}
	}
	combinations := permutation(towns)
	var sum float64
	for _, comb := range combinations {
		for i := 0; i < n-1; i++ {
			left := math.Pow(float64(comb[i].x - comb[i+1].x), 2)
			right := math.Pow(float64(comb[i].y - comb[i+1].y), 2)
			sum += math.Sqrt(left + right)
		}
	}
	fmt.Println(sum / float64(len(combinations)))
}

func remove(ar []*axis, i int) []*axis {
	tmp := make([]*axis, len(ar))
	copy(tmp, ar)
	return append(tmp[0:i], tmp[i + 1:]...)
}
func permutation(ar []*axis) [][]*axis {
	var result [][]*axis
	if len(ar) == 1 {return append(result, ar)}
	for i, a := range ar {
		for _, b := range permutation(remove(ar, i)) {
			result = append(result, append([]*axis{a}, b...))
		}
	}
	return result
}


type IO struct {
	scanner *bufio.Scanner
	writer  *bufio.Writer
}

func NewIO() (*IO, func()) {
	io := &IO{
		scanner: newScanner(),
		writer:  newWriter(),
	}
	return io, func() { io.writer.Flush() }
}

func newScanner() *bufio.Scanner {
	s := bufio.NewScanner(os.Stdin)
	s.Buffer(make([]byte, 10000000), 10000000)
	s.Split(bufio.ScanWords)
	return s
}

func newWriter() *bufio.Writer {
	return bufio.NewWriter(os.Stdout)
}

func (io *IO) ScanBytes() []byte {
	if !io.scanner.Scan() {
		panic("scan string failed")
	}
	return io.scanner.Bytes()
}

func (io *IO) ScanString() string {
	if !io.scanner.Scan() {
		panic("scan string failed")
	}
	return io.scanner.Text()
}

func (io *IO) ScanStrings(n int) []string {
	strs := make([]string, n)
	for i := 0; i < n; i++ {
		strs[i] = io.ScanString()
	}
	return strs
}

func (io *IO) Scan2DStrings(y, x int) [][]string {
	strings := make([][]string, y)
	for i := 0; i < y; i++ {
		strings[i] = io.ScanStrings(x)
	}
	return strings
}

func (io *IO) Scan2DGraph(y int) [][]string {
	strs := make([][]string, y)
	for i := 0; i < y; i++ {
		strs[i] = strings.Split(io.ScanString(), "")
	}
	return strs
}

func (io *IO) ScanInt() int {
	return int(io.ScanInt64())
}

func (io *IO) ScanInt2() (int, int) {
	return int(io.ScanInt64()), int(io.ScanInt64())
}

func (io *IO) ScanInt3() (int, int, int) {
	return int(io.ScanInt64()), int(io.ScanInt64()), int(io.ScanInt64())
}

func (io *IO) ScanInt4() (int, int, int, int) {
	return int(io.ScanInt64()), int(io.ScanInt64()), int(io.ScanInt64()), int(io.ScanInt64())
}

func (io *IO) ScanInts(n int) []int {
	ints := make([]int, n)
	for i := 0; i < n; i++ {
		ints[i] = io.ScanInt()
	}
	return ints
}

func (io *IO) Scan2DInts(y, x int) [][]int {
	ints := make([][]int, y)
	for i := 0; i < y; i++ {
		ints[i] = io.ScanInts(x)
	}
	return ints
}

func (io *IO) ScanInt64() int64 {
	i, err := strconv.ParseInt(io.ScanString(), 10, 64)
	if err != nil {
		panic(err)
	}
	return i
}

func (io *IO) ScanFloat64() float64 {
	i, _ := strconv.ParseFloat(io.ScanString(), 64)
	return i
}

func (io *IO) ScanFloat64s(n int) []float64 {
	floats := make([]float64, n)
	for i := 0; i < n; i++ {
		floats[i] = io.ScanFloat64()
	}
	return floats
}

func (io *IO) Println(a ...interface{}) {
	fmt.Fprintln(io.writer, a...)
}


Measured memory_bytes: 18688