package main

import (
	"bufio"
	"bytes"
	"fmt"
	"io"
	"math"
	"os"
	"strconv"
)

// -----------------------------------------------------------------------------
// IO helper functions

// Returns next token from input.  It must be initialized by SetInput()
// before the key call.
var nextToken func() ([]byte, error)

// OutputBuffer Holds a buffer for output.  It must be initialized by SetOutput().
// All IO fucntions (read*() and [e]print*()) should write to OutputWriter
// instead of this.
var OutputBuffer *bufio.Writer

// OutputWriter Holds an io.Writer.  It must be initialized by SetOutput()
var OutputWriter io.Writer

// SetInteractive SetInteractive Set IO functions for interactive input/output.
func SetInteractive(w io.Writer, r io.Reader) {
	SetUnbefferedInput(r)
	OutputBuffer = nil
	OutputWriter = w
}

// SetOutput Setup OutputBuffer and OutputWriter.
func SetOutput(w io.Writer) {
	OutputBuffer = bufio.NewWriter(w)
	OutputWriter = OutputBuffer
}

// Flush Flushes OutputBuffer
func Flush() {
	if OutputBuffer != nil {
		OutputBuffer.Flush()
	}
}

// IsSpace Returns true if c is a white space
func IsSpace(c byte) bool {
	switch c {
	case '\t', '\n', '\v', '\f', '\r', ' ':
		return true
	}
	return false
}

// SetInput Setup nextToken with input buffer.
func SetInput(r io.Reader) {
	buf := new(bytes.Buffer)
	var b []byte

	var i int
	rest := func() ([]byte, error) {
		for i < len(b) && IsSpace(b[i]) {
			i++
		}
		j := i
		for i < len(b) && !IsSpace(b[i]) {
			i++
		}
		if i == j {
			return nil, io.ErrUnexpectedEOF
		}
		return b[j:i], nil
	}
	initial := func() ([]byte, error) {
		io.Copy(buf, r)
		b = buf.Bytes()
		nextToken = rest
		return rest()
	}
	nextToken = initial
}

// SetUnbefferedInput Setup nextToken without input buffer.
func SetUnbefferedInput(r io.Reader) {
	buf := bufio.NewReader(r)
	var b []byte

	var i int
	nextToken = func() ([]byte, error) {
		var err error
		if i == len(b) {
			b, err = buf.ReadBytes('\n')
			if err != nil {
				return nil, err
			}
			i = 0
			j := len(b) - 1
			for 0 <= j && IsSpace(b[j]) {
				j--
			}
			b = b[0 : j+1]
		}
		for i < len(b) && IsSpace(b[i]) {
			i++
		}
		j := i
		for i < len(b) && !IsSpace(b[i]) {
			i++
		}
		if i == j {
			return nil, io.ErrUnexpectedEOF
		}
		return b[j:i], nil
	}
}

// -----------------------------------------------------------------------------

// IO functions

// Reads next token and return it as []byte
func readb() []byte {
	b, err := nextToken()
	if err != nil {
		panic(err)
	}
	return b
}

// Reads next token and return it as string
func reads() string {
	return string(readb())
}

// Reads next token and return it as int64
func readll() int64 {
	i, err := strconv.ParseInt(reads(), 10, 64)
	if err != nil {
		panic(err.Error())
	}
	return i
}

// Reads next token and return it as int
func readi() int {
	return int(readll())
}

// Reads next token and return it as float64
func readf() float64 {
	f, err := strconv.ParseFloat(reads(), 64)
	if err != nil {
		panic(err.Error())
	}
	return f
}

// Write args to OutputWriter with the format f
func printf(f string, args ...interface{}) (int, error) {
	return fmt.Fprintf(OutputWriter, f, args...)
}

// Write args to OutputWriter without format
func println(args ...interface{}) (int, error) {
	return fmt.Fprintln(OutputWriter, args...)
}

// Write args to stderr with the format f
func eprintf(f string, args ...interface{}) (int, error) {
	return fmt.Fprintf(os.Stderr, f, args...)
}

// Write args to stderr without format
func eprintln(args ...interface{}) (int, error) {
	return fmt.Fprintln(os.Stderr, args...)
}

// -----------------------------------------------------------------------------
// Simple math functions

func min(nums ...int) int {
	if len(nums) == 0 {
		panic("funciton min() requires at least one argument.")
	}
	res := nums[0]
	for i := 0; i < len(nums); i++ {
		res = int(math.Min(float64(res), float64(nums[i])))
	}
	return res
}

func max(nums ...int) int {
	if len(nums) == 0 {
		panic("funciton max() requires at least one argument.")
	}
	res := nums[0]
	for i := 0; i < len(nums); i++ {
		res = int(math.Max(float64(res), float64(nums[i])))
	}
	return res
}

func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}
func abs64(a int64) int64 {
	if a < 0 {
		return -a
	}
	return a
}

// INF 大きな数
const INF int = 1000_000_010

// MOD 余りの計算用
const MOD int = 1000_000_007

// =============================================================================
func init() {
	// for non-interactive
	// SetInput(os.Stdin)
	// SetOutput(os.Stdout)

	// Enable below when interactive.  Its ok to leave above intact.
	SetInteractive(os.Stdout, os.Stdin)
}

func main() {
	defer Flush()

	N := readi()
	X := make([]int, N)
	Y := make([]int, N)
	no := make([]int, N)
	for i := 0; i < N; i++ {
		X[i] = readi()
		Y[i] = readi()
		no[i] = i
	}
	wk, cnt, j, j2 := 0, 0, 0, 0
	var sum float64 = 0
	for {
		for i := 0; i < N-1; i++ {
			j = no[i]
			j2 = no[i+1]
			wk = (X[j]-X[j2])*(X[j]-X[j2]) + (Y[j]-Y[j2])*(Y[j]-Y[j2])
			sum += math.Sqrt(float64(wk))
		}
		cnt++
		if !nextPermutation(no) {
			break
		}
	}
	printf("%.10f", sum/float64(cnt))
}

// 与えられた配列の中身を辞書順に、次の順列をセットする（次がなければ　false を返す）
// 同じ数字が複数ある場合、同じ数列は複数作成しないので注意！
//  →　同じ数列が複数必要な場合は、引数にはインデックスの配列を渡すようにする
//     並びは辞書順ではなくなる
func nextPermutation(a []int) bool {
	for i := len(a) - 2; i >= 0; i-- {
		if a[i] < a[i+1] {
			for j := len(a) - 1; ; j-- {
				if a[i] < a[j] {
					a[i], a[j] = a[j], a[i]
					i++
					for j = len(a) - 1; i < j; j-- {
						a[i], a[j] = a[j], a[i]
						i++
					}
					return true
				}
			}
		}
	}
	return false
}

// =============================================================================


Measured memory_bytes: 1776