import sys

from itertools import permutations

def main():

    input = sys.stdin.readline

    N = int(eval(input()))

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

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

    ans = 0

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

        for i in range(N-1):

            ans += dist(xy[route[i]][0],xy[route[i]][1],xy[route[i+1]][0],xy[route[i+1]][1])

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

        ans /= i

    print(ans)



if __name__ == '__main__':

    main()
import sys

def main():

    input = sys.stdin.readline

    N = int(eval(input()))

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

    def dist(xy1,xy2): return ((xy1[0]-xy2[0])**2 + (xy1[1]-xy2[1])**2)**0.5

    ans = 0

    for i in range(N):

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

            ans += dist(xy[i],xy[j])

    print((ans*2/N))



if __name__ == '__main__':

    main()
import sys
from itertools import permutations


def main():
    input = sys.stdin.readline
    N = int(eval(input()))
    xy = [tuple(map(int, input().split())) for _ in range(N)]

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

    ans = 0
    for route in permutations(list(range(N))):
        for i in range(N - 1):
            ans += dist(
                xy[route[i]][0],
                xy[route[i]][1],
                xy[route[i + 1]][0],
                xy[route[i + 1]][1],
            )
    for i in range(1, N + 1):
        ans /= i
    print(ans)


if __name__ == "__main__":
    main()

import sys


def main():
    input = sys.stdin.readline
    N = int(eval(input()))
    xy = [tuple(map(int, input().split())) for _ in range(N)]

    def dist(xy1, xy2):
        return ((xy1[0] - xy2[0]) ** 2 + (xy1[1] - xy2[1]) ** 2) ** 0.5

    ans = 0
    for i in range(N):
        for j in range(i + 1, N):
            ans += dist(xy[i], xy[j])
    print((ans * 2 / N))


if __name__ == "__main__":
    main()

-from itertools import permutations -    def dist(x1, y1, x2, y2): -        return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5 +    def dist(xy1, xy2): +        return ((xy1[0] - xy2[0]) ** 2 + (xy1[1] - xy2[1]) ** 2) ** 0.5 -    for route in permutations(list(range(N))): -        for i in range(N - 1): -            ans += dist( -                xy[route[i]][0], -                xy[route[i]][1], -                xy[route[i + 1]][0], -                xy[route[i + 1]][1], -            ) -    for i in range(1, N + 1): -        ans /= i -    print(ans) +    for i in range(N): +        for j in range(i + 1, N): +            ans += dist(xy[i], xy[j]) +    print((ans * 2 / N))