from itertools import permutations

from math import factorial



N = int(eval(input()))

coord = []

for _ in range(N):

   x,y = list(map(int, input().split()))

   coord.append((x,y))



def dist(a, b):

   x1, y1 = a

   x2, y2 = b

   return pow((x1-x2) ** 2 + (y1-y2) ** 2, 1/2)



res = 0

div = factorial(N)

idx = [i for i in range(N)]

for line in permutations(idx, N):

   for i in range(N-1):

      res += dist(coord[line[i]], coord[line[i+1]]) / div



print(res)
from itertools import permutations

from math import factorial



N = int(eval(input()))

coord = []

for _ in range(N):

   x,y = list(map(int, input().split()))

   coord.append((x,y))



def dist(a, b):

   x1, y1 = a

   x2, y2 = b

   return pow((x1-x2) ** 2 + (y1-y2) ** 2, 1/2)



res = 0

for i in range(N):

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

      res += dist(coord[i], coord[j]) * 2 / N



print(res)

from itertools import permutations
from math import factorial

N = int(eval(input()))
coord = []
for _ in range(N):
    x, y = list(map(int, input().split()))
    coord.append((x, y))


def dist(a, b):
    x1, y1 = a
    x2, y2 = b
    return pow((x1 - x2) ** 2 + (y1 - y2) ** 2, 1 / 2)


res = 0
div = factorial(N)
idx = [i for i in range(N)]
for line in permutations(idx, N):
    for i in range(N - 1):
        res += dist(coord[line[i]], coord[line[i + 1]]) / div
print(res)

from itertools import permutations
from math import factorial

N = int(eval(input()))
coord = []
for _ in range(N):
    x, y = list(map(int, input().split()))
    coord.append((x, y))


def dist(a, b):
    x1, y1 = a
    x2, y2 = b
    return pow((x1 - x2) ** 2 + (y1 - y2) ** 2, 1 / 2)


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

-div = factorial(N) -idx = [i for i in range(N)] -for line in permutations(idx, N): -    for i in range(N - 1): -        res += dist(coord[line[i]], coord[line[i + 1]]) / div +for i in range(N): +    for j in range(i + 1, N): +        res += dist(coord[i], coord[j]) * 2 / N