import itertools

import math



N = int(eval(input()))

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

cnt = 0

tot = 0

for arr in itertools.permutations(list(range(N))):

    now = XY[arr[0]]

    cnt += 1

    for i in arr[1:]:

        nex = XY[i]

        tot += math.sqrt(pow(now[0]-nex[0], 2)+pow(now[1]-nex[1], 2))

print((tot/cnt))

import itertools

import math



N = int(eval(input()))

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

cnt = 0

tot = 0

for ptn in itertools.permutations(XY):

    now = ptn[0]

    cnt += 1

    for nex in ptn[1:]:

        tot += math.sqrt(pow(now[0]-nex[0], 2)+pow(now[1]-nex[1], 2))

        now = nex

print((tot/cnt))

import itertools
import math

N = int(eval(input()))
XY = [tuple(map(int, input().split())) for _ in range(N)]
cnt = 0
tot = 0
for arr in itertools.permutations(list(range(N))):
    now = XY[arr[0]]
    cnt += 1
    for i in arr[1:]:
        nex = XY[i]
        tot += math.sqrt(pow(now[0] - nex[0], 2) + pow(now[1] - nex[1], 2))
print((tot / cnt))

import itertools
import math

N = int(eval(input()))
XY = [tuple(map(int, input().split())) for _ in range(N)]
cnt = 0
tot = 0
for ptn in itertools.permutations(XY):
    now = ptn[0]
    cnt += 1
    for nex in ptn[1:]:
        tot += math.sqrt(pow(now[0] - nex[0], 2) + pow(now[1] - nex[1], 2))
        now = nex
print((tot / cnt))

-for arr in itertools.permutations(list(range(N))): -    now = XY[arr[0]] +for ptn in itertools.permutations(XY): +    now = ptn[0] -    for i in arr[1:]: -        nex = XY[i] +    for nex in ptn[1:]: +        now = nex