import sys

import itertools

import math

#input = sys.stdin.readline

sys.setrecursionlimit(10**7)

N = int(eval(input()))

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



pattern = list(itertools.permutations(list(range(N))))



pall = 0

for pat in pattern:

    for i in range(N-1):

        currp = xy[pat[i]]

        nextp = xy[pat[i+1]]

        dist = math.sqrt(

            (currp[0] - nextp[0])**2

          + (currp[1] - nextp[1])**2

            )

        pall += dist



print((pall/(len(pattern))))

    

import sys, math

from itertools import permutations, combinations

from collections import defaultdict, Counter, deque

from math import factorial#, gcd

from bisect import bisect_left #bisect_left(list, value)

sys.setrecursionlimit(10**7)

enu = enumerate

MOD = 10**9+7

def input(): return sys.stdin.readline()[:-1]

def pri(x): print(('\n'.join(map(str, x))))



N = int(eval(input()))

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



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

cnt = 0

for route in permutations(list(range(1, N+1))):

    for val1, val2 in zip(route[:-1], route[1:]):

        val1, val2 = val1-1, val2-1

        dist = d[val1][val2]

        if dist > 0:

            cnt += dist

        elif dist == 0:

            d[val1][val2] = (pow(xy[val1][0]-xy[val2][0], 2) + pow(xy[val1][1]-xy[val2][1], 2))**(1/2)

            cnt += d[val1][val2]

res = cnt/factorial(N)

print(res)



import sys
import itertools
import math

# input = sys.stdin.readline
sys.setrecursionlimit(10**7)
N = int(eval(input()))
xy = [list(map(int, input().split())) for _ in range(N)]
pattern = list(itertools.permutations(list(range(N))))
pall = 0
for pat in pattern:
    for i in range(N - 1):
        currp = xy[pat[i]]
        nextp = xy[pat[i + 1]]
        dist = math.sqrt((currp[0] - nextp[0]) ** 2 + (currp[1] - nextp[1]) ** 2)
        pall += dist
print((pall / (len(pattern))))

import sys, math
from itertools import permutations, combinations
from collections import defaultdict, Counter, deque
from math import factorial  # , gcd
from bisect import bisect_left  # bisect_left(list, value)

sys.setrecursionlimit(10**7)
enu = enumerate
MOD = 10**9 + 7


def input():
    return sys.stdin.readline()[:-1]


def pri(x):
    print(("\n".join(map(str, x))))


N = int(eval(input()))
xy = [list(map(int, input().split())) for _ in range(N)]
d = [[0] * (N) for _ in range(N)]
cnt = 0
for route in permutations(list(range(1, N + 1))):
    for val1, val2 in zip(route[:-1], route[1:]):
        val1, val2 = val1 - 1, val2 - 1
        dist = d[val1][val2]
        if dist > 0:
            cnt += dist
        elif dist == 0:
            d[val1][val2] = (
                pow(xy[val1][0] - xy[val2][0], 2) + pow(xy[val1][1] - xy[val2][1], 2)
            ) ** (1 / 2)
            cnt += d[val1][val2]
res = cnt / factorial(N)
print(res)

-import sys -import itertools -import math +import sys, math +from itertools import permutations, combinations +from collections import defaultdict, Counter, deque +from math import factorial  # , gcd +from bisect import bisect_left  # bisect_left(list, value) -# input = sys.stdin.readline +enu = enumerate +MOD = 10**9 + 7 + + +def input(): +    return sys.stdin.readline()[:-1] + + +def pri(x): +    print(("\n".join(map(str, x)))) + + -pattern = list(itertools.permutations(list(range(N)))) -pall = 0 -for pat in pattern: -    for i in range(N - 1): -        currp = xy[pat[i]] -        nextp = xy[pat[i + 1]] -        dist = math.sqrt((currp[0] - nextp[0]) ** 2 + (currp[1] - nextp[1]) ** 2) -        pall += dist -print((pall / (len(pattern)))) +d = [[0] * (N) for _ in range(N)] +cnt = 0 +for route in permutations(list(range(1, N + 1))): +    for val1, val2 in zip(route[:-1], route[1:]): +        val1, val2 = val1 - 1, val2 - 1 +        dist = d[val1][val2] +        if dist > 0: +            cnt += dist +        elif dist == 0: +            d[val1][val2] = ( +                pow(xy[val1][0] - xy[val2][0], 2) + pow(xy[val1][1] - xy[val2][1], 2) +            ) ** (1 / 2) +            cnt += d[val1][val2] +res = cnt / factorial(N) +print(res)