import sys

import math

import fractions

import itertools

from collections import deque

import copy

import bisect



MOD = 10 ** 9 + 7

INF = 10 ** 18 + 7

input = lambda: sys.stdin.readline().strip()



N = int(eval(input()))

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

L = list(itertools.permutations(xy))

ans = 0

for i in L:

    dis = 0

    for l in range(N-1):

        dis += math.sqrt((i[l][0]-i[l+1][0])**2 + (i[l][1] - i[l+1][1])**2)

    ans += dis

print((ans / len(L)))
import sys

import math

from scipy.misc import comb  # atcoderではspecial→misc, permは使えない



MOD = 10 ** 9 + 7

INF = 10 ** 18 + 7

input = lambda: sys.stdin.readline().strip()



N = int(eval(input()))

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

ans = 0

for i in range(N - 1):

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

        ans += math.sqrt((xy[i][0] - xy[l][0]) ** 2 + (xy[i][1] - xy[l][1]) ** 2)

ans *= (N - 1) / comb(N, 2)

print(ans)

import sys
import math
import fractions
import itertools
from collections import deque
import copy
import bisect

MOD = 10**9 + 7
INF = 10**18 + 7
input = lambda: sys.stdin.readline().strip()
N = int(eval(input()))
xy = [list(map(int, input().split())) for _ in range(N)]
L = list(itertools.permutations(xy))
ans = 0
for i in L:
    dis = 0
    for l in range(N - 1):
        dis += math.sqrt((i[l][0] - i[l + 1][0]) ** 2 + (i[l][1] - i[l + 1][1]) ** 2)
    ans += dis
print((ans / len(L)))

import sys
import math
from scipy.misc import comb  # atcoderではspecial→misc, permは使えない

MOD = 10**9 + 7
INF = 10**18 + 7
input = lambda: sys.stdin.readline().strip()
N = int(eval(input()))
xy = [list(map(int, input().split())) for _ in range(N)]
ans = 0
for i in range(N - 1):
    for l in range(i + 1, N):
        ans += math.sqrt((xy[i][0] - xy[l][0]) ** 2 + (xy[i][1] - xy[l][1]) ** 2)
ans *= (N - 1) / comb(N, 2)
print(ans)

-import fractions -import itertools -from collections import deque -import copy -import bisect +from scipy.misc import comb  # atcoderではspecial→misc, permは使えない -L = list(itertools.permutations(xy)) -for i in L: -    dis = 0 -    for l in range(N - 1): -        dis += math.sqrt((i[l][0] - i[l + 1][0]) ** 2 + (i[l][1] - i[l + 1][1]) ** 2) -    ans += dis -print((ans / len(L))) +for i in range(N - 1): +    for l in range(i + 1, N): +        ans += math.sqrt((xy[i][0] - xy[l][0]) ** 2 + (xy[i][1] - xy[l][1]) ** 2) +ans *= (N - 1) / comb(N, 2) +print(ans)