from math import hypot, factorial

from itertools import permutations



N, *XY = list(map(int, open(0).read().split()))



XY = list(zip(*[iter(XY)] * 2))

ans = 0

for p in permutations(XY):

    ans += sum(hypot(ax - bx, ay - by) for (ax, ay), (bx, by) in zip(p, p[1:]))



print(("{:.9f}".format(ans / factorial(N))))

from itertools import combinations

from math import hypot



N, *XY = list(map(int, open(0).read().split()))



print(("{:.9f}".format(sum(hypot(ax - bx, ay - by) for (ax, ay), (bx, by) in combinations(list(zip(*[iter(XY)] * 2)), 2)) * 2 / N)))
from math import hypot, factorial
from itertools import permutations

N, *XY = list(map(int, open(0).read().split()))
XY = list(zip(*[iter(XY)] * 2))
ans = 0
for p in permutations(XY):
    ans += sum(hypot(ax - bx, ay - by) for (ax, ay), (bx, by) in zip(p, p[1:]))
print(("{:.9f}".format(ans / factorial(N))))

from itertools import combinations
from math import hypot

N, *XY = list(map(int, open(0).read().split()))
print(
    (
        "{:.9f}".format(
            sum(
                hypot(ax - bx, ay - by)
                for (ax, ay), (bx, by) in combinations(list(zip(*[iter(XY)] * 2)), 2)
            )
            * 2
            / N
        )
    )
)

-from math import hypot, factorial -from itertools import permutations +from itertools import combinations +from math import hypot -XY = list(zip(*[iter(XY)] * 2)) -ans = 0 -for p in permutations(XY): -    ans += sum(hypot(ax - bx, ay - by) for (ax, ay), (bx, by) in zip(p, p[1:])) -print(("{:.9f}".format(ans / factorial(N)))) +print( +    ( +        "{:.9f}".format( +            sum( +                hypot(ax - bx, ay - by) +                for (ax, ay), (bx, by) in combinations(list(zip(*[iter(XY)] * 2)), 2) +            ) +            * 2 +            / N +        ) +    ) +)