import itertools

import math



N = int(eval(input()))

crd_list = []

for _ in range(N):

    crd_list.append([int(xy) for xy in input().split()])



cost_list = []

for way in itertools.permutations([n for n in range(N)]):

    cost = 0

    for index in range(1, N):

        cost += ((crd_list[way[index]][0]-crd_list[way[index-1]][0])**2 + (crd_list[way[index]][1]-crd_list[way[index-1]][1])**2)**0.5

    cost_list.append(cost)



print((sum(cost_list)/math.factorial(N)))
import sys

import itertools





input = sys.stdin.readline





def fact(n):

    res = 1

    for i in range(n):

        res *= n - i

    return res





def main():

    dst_sum = 0

    for p in itertools.permutations(T):

        for i in range(1, N):

            px, py = p[i-1]

            cx, cy = p[i]

            dst_sum += ((cx-px)**2 + (cy-py)**2)**0.5

    print((dst_sum/fact(N)))





if __name__ == "__main__":

    N = int(eval(input()))

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

    main()

import itertools
import math

N = int(eval(input()))
crd_list = []
for _ in range(N):
    crd_list.append([int(xy) for xy in input().split()])
cost_list = []
for way in itertools.permutations([n for n in range(N)]):
    cost = 0
    for index in range(1, N):
        cost += (
            (crd_list[way[index]][0] - crd_list[way[index - 1]][0]) ** 2
            + (crd_list[way[index]][1] - crd_list[way[index - 1]][1]) ** 2
        ) ** 0.5
    cost_list.append(cost)
print((sum(cost_list) / math.factorial(N)))

import sys
import itertools

input = sys.stdin.readline


def fact(n):
    res = 1
    for i in range(n):
        res *= n - i
    return res


def main():
    dst_sum = 0
    for p in itertools.permutations(T):
        for i in range(1, N):
            px, py = p[i - 1]
            cx, cy = p[i]
            dst_sum += ((cx - px) ** 2 + (cy - py) ** 2) ** 0.5
    print((dst_sum / fact(N)))


if __name__ == "__main__":
    N = int(eval(input()))
    T = [tuple(map(int, input().split())) for _ in range(N)]
    main()

+import sys -import math -N = int(eval(input())) -crd_list = [] -for _ in range(N): -    crd_list.append([int(xy) for xy in input().split()]) -cost_list = [] -for way in itertools.permutations([n for n in range(N)]): -    cost = 0 -    for index in range(1, N): -        cost += ( -            (crd_list[way[index]][0] - crd_list[way[index - 1]][0]) ** 2 -            + (crd_list[way[index]][1] - crd_list[way[index - 1]][1]) ** 2 -        ) ** 0.5 -    cost_list.append(cost) -print((sum(cost_list) / math.factorial(N))) +input = sys.stdin.readline + + +def fact(n): +    res = 1 +    for i in range(n): +        res *= n - i +    return res + + +def main(): +    dst_sum = 0 +    for p in itertools.permutations(T): +        for i in range(1, N): +            px, py = p[i - 1] +            cx, cy = p[i] +            dst_sum += ((cx - px) ** 2 + (cy - py) ** 2) ** 0.5 +    print((dst_sum / fact(N))) + + +if __name__ == "__main__": +    N = int(eval(input())) +    T = [tuple(map(int, input().split())) for _ in range(N)] +    main()