import itertools

import math



N = int(eval(input()))

P = [[int(x) for x in input().split()] for _ in range(N)]

ans=0

for p in itertools.permutations(P):

    for (x1, y1), (x2, y2) in zip(p, p[1:]):

        ans += ((x1-x2)**2 + (y1-y2)**2)**.5

print((ans/math.factorial(N)))
import itertools

import math



N = int(eval(input()))

P = [[int(x) for x in input().split()] for _ in range(N)]

ans=0

for p in itertools.permutations(P):

    for (x1, y1), (x2, y2) in zip(p, p[1:]):

        ans += math.hypot(x1-x2, y1-y2)

print((ans/math.factorial(N)))
import itertools
import math

N = int(eval(input()))
P = [[int(x) for x in input().split()] for _ in range(N)]
ans = 0
for p in itertools.permutations(P):
    for (x1, y1), (x2, y2) in zip(p, p[1:]):
        ans += ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5
print((ans / math.factorial(N)))

import itertools
import math

N = int(eval(input()))
P = [[int(x) for x in input().split()] for _ in range(N)]
ans = 0
for p in itertools.permutations(P):
    for (x1, y1), (x2, y2) in zip(p, p[1:]):
        ans += math.hypot(x1 - x2, y1 - y2)
print((ans / math.factorial(N)))

-        ans += ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5 +        ans += math.hypot(x1 - x2, y1 - y2)