import itertools

import math



def main():

    lists = []

    i=0

    N= int(eval(input()))

    while i < N:

        a = [int(j) for j in input().split()]

        lists.append(a)

        i+=1

    ans=0

    length=0

    for line in itertools.permutations(lists):

        a = []

        b = []

        for data in line:

            if len(b) ==0:

                b = data

                continue

            else:

                a = b

                b = data

                length = math.sqrt(((b[0]-a[0])**2)+((b[1]-a[1])**2))

                ans+=length

    print((ans/math.factorial(N)))



if __name__ == "__main__":

    main()
#-*-coding:utf-8-*-

import itertools

import math



def main():

    lists = []

    N= int(eval(input()))

    lists=[list(map(int,input().split())) for _ in range(N)]

    ans=0

    length=0

    #permutationsは順列を列挙

    for line in itertools.permutations(lists):

        a = []

        b = []

        for data in line:

            if len(b) ==0:

                b = data

                continue

            else:

                a = b

                b = data

                length = math.sqrt(((b[0]-a[0])**2)+((b[1]-a[1])**2))

                ans+=length

    #factorical=階上の値

    print((ans/math.factorial(N)))



if __name__ == "__main__":

    main()
import itertools
import math


def main():
    lists = []
    i = 0
    N = int(eval(input()))
    while i < N:
        a = [int(j) for j in input().split()]
        lists.append(a)
        i += 1
    ans = 0
    length = 0
    for line in itertools.permutations(lists):
        a = []
        b = []
        for data in line:
            if len(b) == 0:
                b = data
                continue
            else:
                a = b
                b = data
                length = math.sqrt(((b[0] - a[0]) ** 2) + ((b[1] - a[1]) ** 2))
                ans += length
    print((ans / math.factorial(N)))


if __name__ == "__main__":
    main()

# -*-coding:utf-8-*-
import itertools
import math


def main():
    lists = []
    N = int(eval(input()))
    lists = [list(map(int, input().split())) for _ in range(N)]
    ans = 0
    length = 0
    # permutationsは順列を列挙
    for line in itertools.permutations(lists):
        a = []
        b = []
        for data in line:
            if len(b) == 0:
                b = data
                continue
            else:
                a = b
                b = data
                length = math.sqrt(((b[0] - a[0]) ** 2) + ((b[1] - a[1]) ** 2))
                ans += length
    # factorical=階上の値
    print((ans / math.factorial(N)))


if __name__ == "__main__":
    main()

+# -*-coding:utf-8-*- -    i = 0 -    while i < N: -        a = [int(j) for j in input().split()] -        lists.append(a) -        i += 1 +    lists = [list(map(int, input().split())) for _ in range(N)] +    # permutationsは順列を列挙 +    # factorical=階上の値