import sys

from math import sqrt

from itertools import permutations

def input(): return sys.stdin.readline().rstrip()

def main():

    n=int(eval(input()))

    A =[[int(_) for _ in input().split()] for i in range(n)]

    a_list=list(permutations(A))

    a_len=len(a_list)

    ans_all=0

    for i in range(a_len):

        keiro = a_list[i]

        ans=0

        x0=keiro[0][0]

        y0=keiro[0][1]

        for x1,y1 in keiro[1:]:

            ans+=sqrt((x1-x0)**2+(y1-y0)**2)

        ans_all+=ans

    print((ans_all/a_len))

if __name__=='__main__':

    main()
import sys

from math import sqrt

def input(): return sys.stdin.readline().rstrip()

def main():

    n=int(eval(input()))

    A =[[int(_) for _ in input().split()] for i in range(n)]

    ans=0

    for i in range(n-1):

        for j in range(i+1,n):

            ans+=sqrt((A[i][0]-A[j][0])**2+(A[i][1]-A[j][1])**2)

    print((ans*2/n))

if __name__=='__main__':

    main()
import sys
from math import sqrt
from itertools import permutations


def input():
    return sys.stdin.readline().rstrip()


def main():
    n = int(eval(input()))
    A = [[int(_) for _ in input().split()] for i in range(n)]
    a_list = list(permutations(A))
    a_len = len(a_list)
    ans_all = 0
    for i in range(a_len):
        keiro = a_list[i]
        ans = 0
        x0 = keiro[0][0]
        y0 = keiro[0][1]
        for x1, y1 in keiro[1:]:
            ans += sqrt((x1 - x0) ** 2 + (y1 - y0) ** 2)
        ans_all += ans
    print((ans_all / a_len))


if __name__ == "__main__":
    main()

import sys
from math import sqrt


def input():
    return sys.stdin.readline().rstrip()


def main():
    n = int(eval(input()))
    A = [[int(_) for _ in input().split()] for i in range(n)]
    ans = 0
    for i in range(n - 1):
        for j in range(i + 1, n):
            ans += sqrt((A[i][0] - A[j][0]) ** 2 + (A[i][1] - A[j][1]) ** 2)
    print((ans * 2 / n))


if __name__ == "__main__":
    main()

-from itertools import permutations -    a_list = list(permutations(A)) -    a_len = len(a_list) -    ans_all = 0 -    for i in range(a_len): -        keiro = a_list[i] -        ans = 0 -        x0 = keiro[0][0] -        y0 = keiro[0][1] -        for x1, y1 in keiro[1:]: -            ans += sqrt((x1 - x0) ** 2 + (y1 - y0) ** 2) -        ans_all += ans -    print((ans_all / a_len)) +    ans = 0 +    for i in range(n - 1): +        for j in range(i + 1, n): +            ans += sqrt((A[i][0] - A[j][0]) ** 2 + (A[i][1] - A[j][1]) ** 2) +    print((ans * 2 / n))