import sys

from itertools import permutations

from math import factorial



read = sys.stdin.read

readline = sys.stdin.readline

readlines = sys.stdin.readlines

sys.setrecursionlimit(10 ** 9)

INF = 1 << 60





def distance2(x1, y1, x2, y2):

    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5





def main():

    N = int(readline())

    C = [tuple(map(int, readline().split())) for _ in range(N)]



    dist = [[0] * N for _ in range(N)]

    for i in range(N):

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

            dist[i][j] = dist[j][i] = distance2(C[i][0], C[i][1], C[j][0], C[j][1])



    ans = 0

    for perm in permutations(list(range(N))):

        for i in range(N - 1):

            ans += dist[perm[i]][perm[i + 1]]



    print((ans / factorial(N)))

    return





if __name__ == '__main__':

    main()

import sys

from itertools import permutations

from math import factorial



read = sys.stdin.read

readline = sys.stdin.readline

readlines = sys.stdin.readlines

sys.setrecursionlimit(10 ** 9)

INF = 1 << 60





def distance2(x1, y1, x2, y2):

    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5





def main():

    N = int(readline())

    C = [tuple(map(int, readline().split())) for _ in range(N)]



    ans = 0

    for i in range(N):

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

            ans += ((C[i][0] - C[j][0]) ** 2 + (C[i][1] - C[j][1]) ** 2) ** 0.5



    print((2 * ans / N))

    return





if __name__ == '__main__':

    main()

import sys
from itertools import permutations
from math import factorial

read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines
sys.setrecursionlimit(10**9)
INF = 1 << 60


def distance2(x1, y1, x2, y2):
    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5


def main():
    N = int(readline())
    C = [tuple(map(int, readline().split())) for _ in range(N)]
    dist = [[0] * N for _ in range(N)]
    for i in range(N):
        for j in range(i + 1, N):
            dist[i][j] = dist[j][i] = distance2(C[i][0], C[i][1], C[j][0], C[j][1])
    ans = 0
    for perm in permutations(list(range(N))):
        for i in range(N - 1):
            ans += dist[perm[i]][perm[i + 1]]
    print((ans / factorial(N)))
    return


if __name__ == "__main__":
    main()

import sys
from itertools import permutations
from math import factorial

read = sys.stdin.read
readline = sys.stdin.readline
readlines = sys.stdin.readlines
sys.setrecursionlimit(10**9)
INF = 1 << 60


def distance2(x1, y1, x2, y2):
    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5


def main():
    N = int(readline())
    C = [tuple(map(int, readline().split())) for _ in range(N)]
    ans = 0
    for i in range(N):
        for j in range(i + 1, N):
            ans += ((C[i][0] - C[j][0]) ** 2 + (C[i][1] - C[j][1]) ** 2) ** 0.5
    print((2 * ans / N))
    return


if __name__ == "__main__":
    main()

-    dist = [[0] * N for _ in range(N)] +    ans = 0 -            dist[i][j] = dist[j][i] = distance2(C[i][0], C[i][1], C[j][0], C[j][1]) -    ans = 0 -    for perm in permutations(list(range(N))): -        for i in range(N - 1): -            ans += dist[perm[i]][perm[i + 1]] -    print((ans / factorial(N))) +            ans += ((C[i][0] - C[j][0]) ** 2 + (C[i][1] - C[j][1]) ** 2) ** 0.5 +    print((2 * ans / N))