import sys

from itertools import permutations

from math import factorial

import numpy as np

int1 = lambda x: int(x) - 1

read = sys.stdin.buffer.read

readline = sys.stdin.buffer.readline

readlines = sys.stdin.buffer.readlines

sys.setrecursionlimit(500000)



N = int(readline())

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



dist = 0

for route in permutations(XY, N):

    acc = 0

    dists = [

        np.sqrt(

            pow(route[i][0] - route[i + 1][0], 2) +

            pow(route[i][1] - route[i + 1][1], 2)) for i in range(N - 1)

    ]

    dist += sum(dists)

print((dist / factorial(N)))
import sys

from itertools import permutations

from math import factorial

int1 = lambda x: int(x) - 1

read = sys.stdin.buffer.read

readline = sys.stdin.buffer.readline

readlines = sys.stdin.buffer.readlines

sys.setrecursionlimit(500000)



N = int(readline())

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



dist = 0

for route in permutations(XY, N):

    acc = 0

    dists = [

        pow(

            pow(route[i][0] - route[i + 1][0], 2) +

            pow(route[i][1] - route[i + 1][1], 2), .5) for i in range(N - 1)

    ]

    dist += sum(dists)

print((dist / factorial(N)))
import sys
from itertools import permutations
from math import factorial
import numpy as np

int1 = lambda x: int(x) - 1
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(500000)
N = int(readline())
XY = [list(map(int, readline().split())) for _ in range(N)]
dist = 0
for route in permutations(XY, N):
    acc = 0
    dists = [
        np.sqrt(
            pow(route[i][0] - route[i + 1][0], 2)
            + pow(route[i][1] - route[i + 1][1], 2)
        )
        for i in range(N - 1)
    ]
    dist += sum(dists)
print((dist / factorial(N)))

import sys
from itertools import permutations
from math import factorial

int1 = lambda x: int(x) - 1
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(500000)
N = int(readline())
XY = [list(map(int, readline().split())) for _ in range(N)]
dist = 0
for route in permutations(XY, N):
    acc = 0
    dists = [
        pow(
            pow(route[i][0] - route[i + 1][0], 2)
            + pow(route[i][1] - route[i + 1][1], 2),
            0.5,
        )
        for i in range(N - 1)
    ]
    dist += sum(dists)
print((dist / factorial(N)))

-import numpy as np -        np.sqrt( +        pow( -            + pow(route[i][1] - route[i + 1][1], 2) +            + pow(route[i][1] - route[i + 1][1], 2), +            0.5,