#!/usr/bin/env python3

from itertools import permutations





def f(i, j):

    return ((x[i] - x[j])**2 + (y[i] - y[j])**2)**0.5





n = int(eval(input()))

x = [None] * n

y = [None] * n

for i in range(n):

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

ans = 0

for s in permutations(list(range(n))):

    tmp = 0

    for i in range(n - 1):

        tmp += f(int(s[i + 1]), int(s[i]))

    ans += tmp

for i in range(n):

    ans /= i + 1

print(ans)

from itertools import permutations





def dfs(p, bit):

    res = 0

    for k in range(n):

        if bit >> k & 1:

            continue

        d = ((x[k] - x[p]) ** 2 + (y[k] - y[p]) ** 2) ** 0.5

        res += d + dfs(k, bit | (1 << k))

    return res





n = int(eval(input()))

x = [0] * n

y = [0] * n

for i in range(n):

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



ans = 0

# for i in range(n):

#     ans += dfs(i, 1 << i)

# for i in range(n):

#     ans /= i + 1

for li in list(permutations(list(range(n)), n)):

    tmp = 0

    for i in range(n - 1):

        p = li[i]

        k = li[i + 1]

        tmp += ((x[k] - x[p]) ** 2 + (y[k] - y[p]) ** 2) ** 0.5

    ans += tmp

for i in range(n):

    ans /= i + 1



print(ans)

#!/usr/bin/env python3
from itertools import permutations


def f(i, j):
    return ((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) ** 0.5


n = int(eval(input()))
x = [None] * n
y = [None] * n
for i in range(n):
    x[i], y[i] = list(map(int, input().split()))
ans = 0
for s in permutations(list(range(n))):
    tmp = 0
    for i in range(n - 1):
        tmp += f(int(s[i + 1]), int(s[i]))
    ans += tmp
for i in range(n):
    ans /= i + 1
print(ans)

from itertools import permutations


def dfs(p, bit):
    res = 0
    for k in range(n):
        if bit >> k & 1:
            continue
        d = ((x[k] - x[p]) ** 2 + (y[k] - y[p]) ** 2) ** 0.5
        res += d + dfs(k, bit | (1 << k))
    return res


n = int(eval(input()))
x = [0] * n
y = [0] * n
for i in range(n):
    x[i], y[i] = list(map(int, input().split()))
ans = 0
# for i in range(n):
#     ans += dfs(i, 1 << i)
# for i in range(n):
#     ans /= i + 1
for li in list(permutations(list(range(n)), n)):
    tmp = 0
    for i in range(n - 1):
        p = li[i]
        k = li[i + 1]
        tmp += ((x[k] - x[p]) ** 2 + (y[k] - y[p]) ** 2) ** 0.5
    ans += tmp
for i in range(n):
    ans /= i + 1
print(ans)

-#!/usr/bin/env python3 -def f(i, j): -    return ((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) ** 0.5 +def dfs(p, bit): +    res = 0 +    for k in range(n): +        if bit >> k & 1: +            continue +        d = ((x[k] - x[p]) ** 2 + (y[k] - y[p]) ** 2) ** 0.5 +        res += d + dfs(k, bit | (1 << k)) +    return res -x = [None] * n -y = [None] * n +x = [0] * n +y = [0] * n -for s in permutations(list(range(n))): +# for i in range(n): +#     ans += dfs(i, 1 << i) +# for i in range(n): +#     ans /= i + 1 +for li in list(permutations(list(range(n)), n)): -        tmp += f(int(s[i + 1]), int(s[i])) +        p = li[i] +        k = li[i + 1] +        tmp += ((x[k] - x[p]) ** 2 + (y[k] - y[p]) ** 2) ** 0.5